JP4819183B2 - Rail welded portion cooling method, rail welded portion cooling device, and rail welded joint - Google Patents

Description

The present invention relates to a rail welded portion cooling method, a rail welded portion cooling apparatus, and a rail welded joint that improve the fatigue strength of the welded joint as compared with the prior art. In particular, the present invention relates to a cooling method and a cooling device for a rail joint immediately after welding.
This application claims priority based on Japanese Patent Application No. 2009-081587 filed in Japan on March 30, 2009 and Japanese Patent Application No. 2009-175646 filed on July 28, 2009 in Japan. And the contents thereof are incorporated herein.

  Rail joints (rail welds) are most likely to be damaged among the rails and require maintenance costs. The rail joint is the main source of noise and vibration generated when the train passes. Since speeding up and heavy loading of trains are being promoted in various places, the technology of making rail joints having the above-mentioned problems continuous by welding to make long rails is becoming common.

  A general rail will be described with reference to FIGS. 1A and 1B. FIG. 1A is a side view of a long rail. Long rails are manufactured by welding at least two rails. For this reason, the welded portion 7 is included in the long rail. A weld bead 8 is present at the weld 7.

  1B is a cross-sectional view taken along line AA shown in FIG. 1A. As shown in FIG. 1B, the rail exists between the head 1 (the upper part of the rail) in contact with the wheels, the foot 3 (the lower part of the rail) provided on the sleepers, and the head 1 and the foot 3. It has a column part 2. The head 1 has a top 4 and the foot has a front surface 5 and a sole 6.

  Flash butt welding (for example, Patent Document 1), gas pressure welding (for example, Patent Document 2), enclosed arc welding (for example, Patent Document 3), and thermite welding (for example, Patent Document 4) are the main rail welding methods.

  2A to 2C are explanatory diagrams of flash butt welding. In flash butt welding, as shown in FIGS. 2A to 2C, a voltage is applied to the workpiece 10 placed opposite to each other through the electrode 9 to generate an arc between the end faces, and the end face of the workpiece to be welded. Melt. Then, when the workpiece is sufficiently heated, the material is pressed in the axial direction to join the workpiece.

  3A and 3B are explanatory views of thermite welding. 3B is a cross-sectional view taken along line BB shown in FIG. 3A. In thermite welding, as shown in FIG. 3A and FIG. 3B, the material to be welded 10 is provided with a gap of 20 to 30 mm and opposed, and the gap is surrounded by a mold 14. And the molten steel 16 produced | generated by the reaction of aluminum and iron oxide in the crucible 15 is inject | poured in the casting_mold | template, a rail end surface is fuse | melted, and it welds.

  4A to 4C are explanatory diagrams of gas pressure welding. In the gas pressure welding, as shown in FIG. 4A, the welded material in the vicinity of the joint surface is heated from the side by the burner 17 in a state where the joint surface is pressurized, and the joint surface is pressure-welded at a high temperature. As shown in FIG. 4B, the vicinity of the weld is expanded and deformed by pressurization. Then, as shown in FIG. 4C, the expanded portion is removed by the trimmer 18.

  5A and 5B are explanatory diagrams of the enclose arc welding. As shown in FIGS. 5A and 5B, in the enclosed arc welding, the workpieces are made to face each other with a gap of 10 to 20 mm, and the backing metal 19 and the side surface plating 20 are arranged around the obvious area. Then, the welding metal is raised using the welding rod 21 in the meantime. This method is a so-called manual arc welding method.

  In particular, on a route through which a heavy-duty freight train passes or a route in a cold region, fatigue cracks may occur starting from the neutral axis of the rail weld. Therefore, in order to prevent brittle fracture of the rail due to this fatigue crack, it is necessary to frequently replace the rail. An example of this state is shown in FIGS. 6A and 6B.

  FIG. 6A is a diagram showing fatigue cracks 22 generated in the horizontal direction in the column portion and brittle cracks 23 resulting therefrom. Moreover, FIG. 6B is a figure which shows the crack surface of the fatigue crack 22 and the brittle crack 23 which are shown to FIG. 6A. The fatigue crack 22 is generated in the horizontal direction starting from a welding defect near the weld bead 8 near the neutral axis. After the brittle crack 23 caused by the fatigue crack 22 penetrates the column portion in the thickness direction, one crack progresses to the rail head portion side and the other crack progresses to the foot portion side. The starting point of the fatigue crack 22 is not limited to a weld defect, and various factors can be considered.

  The occurrence of fatigue cracks is thought to be influenced by the residual stress inside the material as well as external load conditions. FIG. 7A shows the residual stress distribution in the circumferential direction at the periphery of the rail weld. In FIG. 7A, when the residual stress is greater than 0, it indicates that a tensile residual stress exists, and when the residual stress is less than 0, it indicates that a compressive residual stress exists.

  FIG. 7A shows that a large tensile residual stress in the rail circumferential direction (that is, the vertical direction) is generated by welding in the vicinity of the column portion of the rail welded portion. Therefore, it is considered that a fatigue crack starting from a weld defect occurs because a repeated load is applied to the vicinity of a column portion of a rail welded portion having a large tensile residual stress due to passage of a train. In order to prevent such fatigue cracks, it is desirable to prevent the weld defect as a starting point and to render it harmless even if a weld defect exists.

  FIG. 7B shows the relationship between the distance from the welding center (rail length direction) and the residual stress existing in the vertical direction of the column portion of the rail. It can be seen from FIG. 7B that there is a large tensile residual stress in a range from the welding center to about 25 mm.

  A track in a railway has a rail and sleepers that support the rail. When the train passes on the rail, a load distributed from the wheels of many trains is applied to the rail.

  The cause of the above-mentioned fatigue crack is related to the load state from the wheel on the rail weld. The rail load when the train passes is different between the rail portion directly above the sleepers 24 and the rail portion between the two sleepers 24. A vertical load of a train is directly applied to the rail portion directly above the sleepers 24. When a long rail welded at a factory is installed on a sleeper locally, the position of the welded portion and the sleeper may coincide by chance. It is considered that there are several places where the sleeper position and the welded part coincide with each other on a long rail of several hundred meters.

  FIG. 9A shows a point in time when the wheel 25 passes directly above the sleeper 24 (welded part) at a location where the position of the sleeper 24 matches the welded part. In this case, the largest stress is generated in the rail pillar portion 2 having a small cross-sectional area. The stress in this case is a compressive stress, but as described above, a large tensile residual stress exists in the rail column portion 2. Therefore, the rail column part 2 is substantially in a state in which repeated stress acts in a state where it receives tensile stress.

On the other hand, FIG. 9B shows a point in time when the wheel 25 passes between the two sleepers 24, 24 (welded part) at a position where the position of the sleeper 24 does not coincide with the welded part. In this case, a load that pushes and bends the wheel 25 from above is applied to the rail. For this reason, a longitudinal compressive stress is generated in the rail head 1, and a longitudinal tensile stress is generated in the rail foot 3. The bending stress applied to the rail column 2 is neutral. Since the tensile stress of the rail foot 3 is generated every time the wheel 25 passes, the rail foot 3 needs to be considered for the occurrence of fatigue cracks.
FIG. 8 shows the residual stress in the longitudinal direction at the periphery of the welded portion by flash butt welding. As shown in the figure, a strong compressive stress remains in the longitudinal direction at the bottom of the rail. For this reason, even if a tensile stress is applied to the rail bottom when the train passes, the effective stress state is offset by the compressive residual stress. Therefore, the occurrence of fatigue cracks can be suppressed. For this reason, there are few examples of fatigue failure from the rail foot, but when the compressive residual stress is small, etc., as shown in FIG. 10A and FIG. It may occur.

In Patent Documents 5 and 6, in order to prevent damage to the rail column portion, a method of bringing the entire rail welded portion or the head portion and the column portion of the rail welded portion to a high temperature state by welding heat or heating from outside, and then performing accelerated cooling. Disclosure. According to this technique, since the residual stress of the rail welded portion is controlled, the tensile residual stress generated in the vertical direction at the column portion of the rail welded portion can be reduced or changed to a compressive residual stress. For this reason, the fatigue strength of a rail welding part can be improved. Such a technique can reduce the occurrence of fatigue cracks from the rail post.
In addition, as a technique for improving the fatigue strength of the rail welded portion, there are a method using shot peening, a method using hammer peening, a grinder process, TIG dressing, and the like as in Patent Document 7, for example.

Moreover, patent document 8 is disclosing the cooling device of a rail welding part.
In order to improve the durability of the long rail, it is necessary to suppress the occurrence of fatigue cracks from the column part and the foot part of the welded part and to make the fatigue resistance characteristics of these parts compatible.

  When the head and the column of the rail welded portion are accelerated and cooled by the cooling method described in Patent Document 5 and Patent Document 6, the tensile residual stress in the vertical direction in the rail column is improved, and the fatigue of the column is thereby improved. It has been shown that the occurrence of cracks is suppressed. However, Non-Patent Document 1 shows that according to this method, the residual stress in the rail longitudinal direction at the sole portion is changed to the tensile residual stress. In recent years, since heavy-duty trains tend to increase, loads due to bending loads on the soles have increased. Since the foot sole is pulled in the longitudinal direction of the rail by the load due to the bending load, the fatigue strength of the rail sole has an important meaning. As described above, the residual stress in the rail longitudinal direction strongly affects the fatigue strength of the rail sole. However, as described above, in the cooling process of Patent Document 5 and Patent Document 6, the residual stress in the rail longitudinal direction of the rail sole is reduced (trying to shift to the tensile residual stress), and thus there is a concern that the fatigue strength may be reduced. The For this reason, there exists a possibility that damage as shown to FIG. 10A and FIG. 10B may generate | occur | produce.

  On the other hand, according to the shot peening process, which is a conventional technique for improving residual stress by mechanical post-processing (that is, for imparting compressive residual stress), a steel ball having a diameter of several millimeters is hit against the material. Fatigue strength can be improved by plastically deforming the surface layer to cause work hardening and increasing the residual stress. However, the treatment requires large-scale equipment for steel ball projection, steel ball collection, dust prevention, etc., and its application is limited to large welds. In addition, it is disadvantageous in cost because it is necessary to replenish the wear and damage of the projection material.

  In addition, with regard to hammer peening that hits the tip of the tool against the material and plastically deforms the weld, it is said that the fatigue strength of the material is improved by introducing compressive stress and reducing stress concentration by plastic deformation. Yes. However, the vibration at the time of striking is large and the burden on the worker is large. In addition, fine control is difficult and uniform processing is difficult. Non-Patent Document 2 shows that wrinkle-like grooves produced by processing are affected depending on processing conditions, and the effect of improving fatigue strength is small.

  Moreover, the grinder process can be expected to have a certain effect by reducing the stress concentration by smoothing the weld bead toe. However, when it cuts too much, since the thickness of the welded portion is insufficient and the strength is reduced, there is a drawback that it requires skill for processing and requires a long time for work.

  Further, the TIG dressing can improve the fatigue strength by reducing the stress concentration by remelting the toe portion of the weld bead with an arc generated from a tungsten electrode and resolidifying it into a smooth shape. However, manual welding in high carbon materials such as rails tends to produce a hard and brittle martensite structure, and strict construction management is necessary to prevent this.

  Further, by using the welded part cooling device disclosed in Patent Document 8, the hardness of the welded part can be increased by performing appropriate cooling from a high temperature state after welding. On the other hand, according to the study by the present inventors, it is necessary to cool an appropriate range with an appropriate strength in order to control the residual stress state of the weld. Although it seems that residual stress changes also by using the apparatus of patent document 8, the cooling conditions for obtaining appropriate residual stress distribution are not demonstrated.

  As described above, the rail joint portion (rail welded portion) is most likely to be damaged among the rails, and requires a maintenance cost. The rail joint is the main source of noise and vibration generated when the train passes. Since speeding up and heavy loading of trains are being promoted in various places, the technology of making rail joints having the above-mentioned problems continuous by welding to make long rails is becoming common.

  Flash butt welding (for example, see Patent Document 1), gas pressure welding (for example, see Patent Document 2), enclose arc welding (for example, see Patent Document 3), and thermite welding (for example, see Patent Document 4) are the main rail weldings. Is the method.

  When the rail joint portion is welded, fatigue cracks may occur near the neutral axis of the rail welded portion, particularly on a route through which a heavy-duty freight train passes or a route in a cold region. For this reason, it is necessary to frequently replace the rail in order to prevent brittle fracture of the rail due to this fatigue crack. 41A and 41B show an example of this state. FIG. 41A shows a state in which a fatigue crack 151 generated in the horizontal direction is generated near the neutral axis of the rail welded portion 150. The brittle crack 152 occurs toward the rail head portion and the rail foot portion. FIG. 41B shows fracture surfaces of the fatigue crack 151 and the brittle crack 152. From FIG. 41B, it can be seen that the fatigue crack 151 occurs starting from the vicinity of the neutral axis of the rail welded portion 150, and then the brittle crack 152 penetrates the column portion in the plate thickness direction. In the present specification, the rail upper portion 160 that contacts the wheel is referred to as a “head”, the rail lower portion 162 that contacts the sleeper is referred to as a “foot”, and a portion 161 between the head and the foot is referred to as a “post”. (See FIGS. 27A and 27B).

  The occurrence of fatigue cracks is considered to be affected by the residual stress inside the material as well as external load conditions. FIG. 42 shows the residual stress distribution in the circumferential direction of the rail welded portion by flash butt welding. In the graph of FIG. 42, the positive direction on the vertical axis represents the tensile residual stress, and the negative direction on the vertical axis represents the compressive residual stress. FIG. 42 shows that the tensile residual stress of the column part is large. When the rail welding part is located on the sleeper, the vertical compressive stress acts on the pillar part when the train passes. However, since a large tensile stress in the vertical direction (rail cross-section circumferential direction) remains in the column portion, the column portion is in a state in which repeated stress is actually applied in a state where the column portion is subjected to the tensile stress. For this reason, fatigue cracks are likely to occur in the column portion.

  In Patent Documents 5 and 6, in order to prevent damage to the rail column portion, a method of bringing the entire rail welded portion or the head portion and the column portion of the rail welded portion to a high temperature state by welding heat or heating from outside, and then performing accelerated cooling. Disclosure. According to this technique, since the residual stress of the rail welded portion is controlled, the tensile residual stress generated in the vertical direction at the column portion of the rail welded portion can be reduced or changed to a compressive residual stress. For this reason, the fatigue strength of a rail welding part can be improved.

  Further, as a technique for improving the fatigue strength of the rail welded portion, there is a method using a shot peening process as disclosed in Patent Document 7, for example. In the shot peening process, a steel ball having a diameter of several millimeters is projected onto the material, and the material surface layer is plastically deformed to be work-hardened. Thereby, the fatigue strength is improved by changing the residual stress to the compressive stress.

  Patent Document 8 cools the air chamber that cools the top surface of the rail welded portion, the air chamber that cools the head side surface of the rail welded portion, and the abdomen (column portion) and bottom (foot) of the rail welded portion. An apparatus for cooling a rail weld having an air chamber is disclosed. Each air chamber is provided with a plurality of nozzles for discharging compressed air, and a nozzle for temperature detection is provided at the center of the nozzle group in the air chamber for cooling the top of the head.

  The rail head is worn by contact with the wheels. In particular, in a curved track, wear is promoted by relative slip generated between the wheel and the rail. For this reason, the heat-treated rail which hardened the rail head part is employ | adopted for a curve area in many cases. In the welding of heat-treated rails, it is desirable that the rail head is accelerated and cooled in the temperature range from the austenite temperature range to the completion of pearlite transformation after welding to obtain the same hardness as the base material to be welded. When accelerating and cooling the rail head after welding, the head and columns of the rail welded part are accelerated to reduce the vertical residual stress in the rail column (that is, the compressive residual stress increases) Non-Patent Document 1 shows that this can suppress the occurrence of fatigue cracks in the column portion. However, it has been found by experiments by the present inventors that the residual stress in the column part is not greatly reduced even if the head and column part of the rail welded part are accelerated and cooled.

  Further, in the case of shot peening, steel balls are projected and collected, and a large-scale facility for preventing dust is necessary, which limits the application to large-sized welds. In addition, since the steel balls are worn out and broken, there is a problem that it is necessary to replenish the steel balls regularly, which increases the running cost.

  Furthermore, when the rail welded part is accelerated and cooled using the cooling device shown in Patent Document 8, the present inventors have found that the residual stress of the rail column part does not decrease and the fatigue life does not extend so much. Determined by the tests performed. That is, it has been clarified that the residual stress of the rail welded portion cannot be reduced (the compressive residual stress can be increased) unless the appropriate range of the rail welded portion is cooled at an appropriate cooling rate.

JP-A-56-136292 JP 11-270810 A JP-A-6-292968 JP-A-48-95337 JP 59-93837 A JP 59-93838 A JP-A-3-249127 JP-A-60-33313

Proceedings of the Second International Conference on residual stresses, ICR2, Nancy, France, 23-25, Nov, 1988, P912-918 Miki, Ami, Tani, Sugimoto, “Improvement of fatigue strength by improving weld toe”, Journal of the Japan Welding Society, Vol. 17, no. 1, P111-119 (1999)

  As described above, there has conventionally been no technique for efficiently increasing the fatigue strength of the rail column portion, the fatigue strength of the rail foot portion, and the hardness of the rail head portion. Therefore, a first object of the present invention is to provide a method for efficiently manufacturing a rail having a fatigue strength of a welded portion which is improved as compared with the conventional art.

  In addition, the second object of the present invention is to ensure sufficient rail head hardness and further reduce the residual stress of the column portion (that is, increase the compressive residual stress). It is providing the cooling method of a rail welding part which can improve the fatigue strength of a part, and the cooling device used for it.

The present invention employs the following means in order to solve the above-described problems.
(1) A first aspect of the present invention is a rail welding having an Ac1 region heated to a transformation start temperature Ac1 or higher of pearlite to austenite and an Ac3 region heated to a transformation completion temperature Ac3 or higher. A first column cooling step for cooling a column cooling region in the rail welded portion in a partial temperature range until the transformation from austenite to pearlite is completed; A second column cooling step for cooling the column portion cooling region after the entire column portion in the section is transformed into pearlite; a foot cooling step for cooling the foot portion in the rail welded portion; and the rail welded portion A head cooling step for cooling the head in the above, and when the cooling time of the first column cooling step and the second column cooling step is t (minutes), the column cooling region The width L of the rail longitudinal direction around the weld, made from the Ac1 region and the Ac3 region, rail longitudinal width LAc1 around the weld region maximum heating temperature immediately after welding is Ac1 or This is a method of cooling a rail welded portion that satisfies the equation represented by k value obtained by dividing −0.1t + 0.63 ≦ k ≦ −0.1t + 2.33.
(2) In the rail welding part cooling method according to (1) above, in the first column part cooling step, cooling is performed at a cooling rate exceeding a natural cooling rate and 5 ° C./s or less. In the column part cooling step, the cooling may be performed at a cooling rate that exceeds the natural cooling rate and is equal to or higher than the cooling rate of the feet.
(3) In the rail welding part cooling method according to (1) above, in the second column part cooling step, cooling is performed at a cooling rate that exceeds the natural cooling rate and is equal to or higher than the cooling rate of the feet. Also good.
(4) In the rail welding portion cooling method according to (1) above, in the first column portion cooling step, the cooling may be performed at a cooling rate exceeding 5 ° C./s, exceeding the natural cooling rate.
(5) In the rail welded portion cooling method according to (1) above, the first column portion cooling step is a first column portion cooling previous step, which is a cooling step in the austenite temperature region, and subsequently to pearlite. And a first column part cooling latter stage step of cooling in a temperature range until the transformation of the first column part is completed. In the first column part cooling first stage step, a natural cooling rate is exceeded and the rail foot part cooling rate is The cooling is performed at the above cooling rate, the natural cooling rate or the cooling rate of 2 ° C./s or less is cooled in the first column cooling later step, and the natural cooling rate is exceeded in the second column cooling step. And you may cool at the cooling rate more than the cooling rate of the said rail leg part.
(6) In the rail welding part cooling method according to (1) above, the cooling rate of the foot may be a natural cooling rate.
(7) In the method for cooling a rail welded portion described in (1) above, in the head cooling step, at least a part of the temperature until the transformation from the austenite temperature range exceeding A3, Ae, or Acm to pearlite is completed. beyond the natural cooling rate in the range may be cooled below the cooling rate of 5 ° C. / s.
(8) In the method for cooling a rail welded portion according to any one of (1) to (7), the cooling of the lower corner portion of the jaw portion is performed when the head portion and the column portion are cooled. The speed may be slower than the cooling rate of the column part.
(9) In the method for cooling a rail welded portion described in (8) above, assuming that the height of the head side portion forming the side surface of the head is Hs, the position is 2Hs / 3 below from the upper end of the head side portion. The entire head except for the lower head region may be accelerated and cooled.
(10) In the method for cooling a rail welded portion described in (9) above, a shielding plate is provided in the head region below the position 2Hs / 3 below the upper end of the head side portion, and directed toward the head. A cooling fluid may be ejected.
(11) A second aspect of the present invention is a rail welded joint cooled using the method for cooling a rail welded portion according to (1) above, wherein the column has a vertical residual stress of 350 MPa or less. A rail welded joint comprising: a portion of the rail foot where the longitudinal residual stress is a compressive stress; and the welded portion in which 95% or more of the metal structure is a pearlite structure.
(12) A third aspect of the present invention is a rail welded joint cooled using the method for cooling a rail welded portion according to (8) above, wherein the column has a residual stress in the vertical direction of 300 MPa or less. A rail welded joint comprising: a portion; and the head having a hardness of Hv320 or more.
(13) In the fourth aspect of the present invention, when the height of the head side portion forming the side surface of the head of the rail welded portion is Hs, the position is lower than the position 2Hs / 3 below the upper end of the head side portion. A head cooling unit that accelerates and cools the entire head except for the head region may be provided.
(14) In the rail welded portion cooling apparatus according to (13), the head cooling unit includes: an ejection portion that ejects a cooling fluid toward the head; 2Hs / from the upper end of the head side portion; And a shielding plate that covers the head region below the lower position.

  According to the methods described in the above (1) to (7), fatigue cracks are generated in the welded part by improving the residual stress of the column part of the rail welded part and controlling the residual stress of the sole part in the compression range. It can be made difficult to occur.

According to the method as described in said (8)-(10), when accelerating and cooling the head part and column part of a rail welded part, a cooling rate of a jaw part is made slower than the cooling rate of a column part, It is possible to further reduce the residual stress of the column part while ensuring sufficient hardness of the head part. For this reason, the wearability of a rail head part and the fatigue strength of a rail welding part can be improved.
According to the rail welded joint described in (11) above, damage due to metal fatigue can be suppressed even when a heavy-duty train passes on the rail.
According to the rail welded joint described in (12) above, damage due to metal fatigue and wear of the rail head can be suppressed even when a heavy-duty train passes on the rail.
According to the devices described in (13) and (14) above, the head cooling unit accelerates and cools the entire head area excluding the head area below the position 2Hs / 3 below the upper end of the head side part. Therefore, the cooling rate of the jaw portion is relaxed, and the cooling rate of the jaw portion can be made slower than the cooling rate of the column portion. For this reason, it is possible to further reduce the vertical residual stress of the column portion while keeping the hardness of the rail head in contact with the wheel high.

It is a side view of a rail. It is sectional drawing obtained along the AA line of FIG. 1A. It is the schematic which shows the flushing process of flash butt welding. It is the schematic which shows the upset process of flash butt welding. It is the schematic which shows the trimming process of flash butt welding. It is the schematic for demonstrating thermite welding. It is sectional drawing obtained along the BB line of FIG. 3A. It is the schematic which shows the heating process of gas pressure welding. It is the schematic which shows the pressurization process of gas pressure welding. It is the schematic which shows the trimming process of gas pressure welding. It is the schematic for demonstrating enclosed arc welding. It is sectional drawing for demonstrating enclose arc welding. It is the schematic which shows the fatigue damage from the column part of a rail welding part. It is the schematic which shows the fracture surface of the damage. It is a graph which shows the residual stress distribution which exists in the circumferential direction of the peripheral part of a rail welding part. It is a graph which shows the relationship between the distance from a welding center, and the residual stress which exists in the up-down direction of a rail pillar part. It is a graph which shows the residual stress distribution which exists in the rail longitudinal direction of the peripheral part of a rail welding part. It is the schematic which shows the time of a wheel passing directly over a sleeper. It is the schematic which shows the time of a wheel passing between sleepers. It is the schematic which shows the fatigue damage from the foot | leg part of a rail welding part. It is the schematic which shows the fracture surface of the damage. It is an equilibrium diagram of carbon steel. It is the schematic which shows the structure | tissue change accompanying the heating and cooling of carbon steel. It is a CCT figure of steel of hypoeutectoid composition. It is a CCT figure of steel of eutectoid composition. It is a CCT figure of the steel of a hypereutectoid composition. It is a graph which shows rail temperature distribution and hardness distribution of a rail axial direction. It is a figure which shows the temperature distribution immediately after welding in the column part of a welding part. It is a figure which shows the temperature distribution and shrinkage | contraction stress at the time of the cooling process in the pillar part of a welding part. It is a figure which shows the temperature distribution in a certain time of the natural cooling process and the accelerated cooling process in the column part of a welding part. It is a figure which shows the temperature distribution in the time of the temperature of the welding center in the column part of a welding part in a natural cooling process and an accelerated cooling process slightly higher than Ar1. It is the schematic which shows temperature distribution at the time of cooling a pillar part extensively. It is the schematic which shows temperature distribution at the time of cooling a leg part excessively. It is the schematic which shows the temperature log | history in the case of accelerated cooling after the completion of a pearlite transformation of a rail pillar part. (First cooling pattern) It is the schematic which shows the temperature history in the case of accelerated cooling of a rail pillar part from the start of austenite decomposition to completion of pearlite transformation. (Second cooling pattern) It is another schematic diagram of the temperature history when the rail column part is accelerated and cooled from the start of austenite decomposition until the completion of pearlite transformation. (Second cooling pattern) It is another schematic diagram of the temperature history when the rail column part is accelerated and cooled from the start of austenite decomposition until the completion of pearlite transformation. (Second cooling pattern) It is the schematic of the temperature history at the time of accelerating cooling the rail pillar part from the start of austenite decomposition to completion of pearlite transformation and further accelerating cooling after completion of pearlite transformation. (Third cooling pattern) It is the schematic of the temperature history at the time of accelerating cooling the rail pillar part and the head from the start of austenite decomposition until the completion of pearlite transformation and further cooling the rail pillar part after completion of the pearlite transformation. (4th cooling pattern) It is the schematic of the temperature history at the time of cooling, and is a figure in the case of providing the gentle cooling area of 2 degrees C / s or less in the pearlite transformation temperature area in the middle of cooling. (4th cooling pattern) It is the schematic of the temperature history at the time of cooling, and is a figure in the case of accelerating cooling the rail column part and the head from the start of austenite decomposition to the completion of pearlite transformation and further accelerating cooling the rail column part after completion of pearlite transformation. (5th cooling pattern) It is the schematic of the temperature distribution of a welding part when a cooling width is narrow. It is the schematic of the temperature distribution of the welding part in case a cooling width is a middle grade. It is the schematic of the temperature distribution of the welding part in case a cooling width is wide. It is the schematic which shows the relationship between k value (ratio of a cooling width | variety and the material width | variety heated to Ac1 or more) and a residual stress in the case of short-time cooling. It is the schematic which shows the relationship between k value (ratio of the cooling width and the material width heated to Ac1 or more) and residual stress in the case of long-time cooling. It is the schematic which shows the relationship between k value (ratio of the cooling width and the material width heated to Ac1 or more) effective for residual stress reduction, and cooling time. It is the schematic of the fatigue strength evaluation test of a column part. It is the schematic of the bending fatigue strength evaluation test of a foot | leg part. It is a side view of a rail. It is sectional drawing obtained along the X'-X 'line | wire of FIG. 27A. It is a fragmentary sectional view of a rail head. It is the schematic of the cooling device of the rail welding part which concerns on one Embodiment of this invention. It is a top view of the head cooling unit which accelerates and cools the head of a rail welding part. It is a side view of the same head cooling unit. It is sectional drawing obtained along the A'-A 'line | wire of FIG. 30A. FIG. 32 is a cross-sectional view taken along line C′-C ′ in FIG. 31. It is sectional drawing obtained along the B'-B 'line | wire of FIG. 30A. It is the schematic which shows the flushing process of flash butt welding. It is the schematic which shows the upset process of flash butt welding. It is the schematic which shows the trimming process of flash butt welding. It is the schematic which shows a temperature log | history when a rail welded part is acceleratedly cooled with the cooling method of the rail welded part which concerns on one Embodiment of this invention. It is the schematic which shows the temperature history when the rail welding part is naturally cooled. It is the schematic which shows a temperature history when only the head of a rail welding part is accelerated-cooled. It is the schematic which shows the temperature history when the head part and pillar part of a rail welding part are accelerated-cooled by the conventional method. It is the schematic which shows an example of the residual stress distribution by four kinds of cooling methods. It is the schematic which showed the measurement location of temperature, hardness, and residual stress in the cooling test of a rail welding part. It is the schematic which shows the fatigue damage from the column part of a rail welding part. It is the schematic which shows the fracture surface of the damage. It is a graph which shows the residual stress distribution which exists in the circumferential direction of the peripheral part of a rail welding part.

<Welding method>
In the present invention, the welding method of the rail welded portion is not limited to flash butt welding. Hereinafter, flash butt welding will be described in more detail with reference to FIGS. 2A to 2C as an example of a method of welding the rail welded portion.
The first step of the flash butt welding method is a step of generating an arc continuously between the end faces shown in FIG. 2A (flushing step). In this step, an arc is generated between the end faces of the material to be welded by the voltage applied via the electrode 9. The portion where the arc is generated is melted locally, and a part of the melted metal is released to the outside as a sputter, and the rest remains on the end face. A dent called a crater occurs in the part melted by the arc. The material to be welded is gradually brought closer, and arcs are successively generated at new contact portions, and the material is gradually shortened by repeated local melting. In this process, the moving speed of the material to be welded is adjusted so that the material interval is kept substantially constant.

  In the middle of the flushing step, there may be a case in which a material end surface is intentionally brought into contact and the temperature of the base material to be welded is increased by a large current by direct energization. The purpose is to smoothen the temperature distribution in the vicinity of the end face and proceed to the upset process more efficiently. This process is called a “preheating process”, and it is usual to repeat contact energization for about 2 to 5 seconds and non-contact and rest periods for about 1 second several times.

  By continuing the flushing process for several tens of seconds to several minutes, the entire end face of the workpiece is melted. Further, the material in the vicinity of the end surface softens due to the temperature rise. When this state is reached, pressure is applied in the axial direction as shown in FIG. 2B. By this pressurization called upset, the crater uneven surface formed on the end faces is crushed, and the molten metal existing between the end faces is pushed out of the system. In the vicinity of the softened end surface, the cross section increases due to plastic deformation, and a weld bead 11 is formed around the weld surface.

  As shown in FIG. 2C, the weld bead 11 is sheared and removed by the trimmer 12 during a high temperature period immediately after welding (trimming step). After trimming, a thin weld bead with a height of several mm and a width of about 10 to 30 mm remains around the weld.

  The thin weld bead remaining after trimming is smoothed and polished by a grinder at the rail head that contacts the wheel. The rail bead and foot weld beads have different treatment methods depending on the railway company, such as complete smoothing by grinder polishing, thinning by grinder polishing, no maintenance.

<Rail material>
Next, rail steel will be described. Rail steel is hypoeutectoid steel containing 0.5 to 0.8% by mass of carbon, or eutectoid carbon containing about 0.8% by mass of carbon, as defined in JIS-E1101 and JIS-E1120. Steel is generally used. Recently, rail steels with a hypereutectoid composition containing more than 0.8% by mass of carbon with improved wear resistance have been spreading for heavy-duty cargo lines in overseas mining railways.

<Equilibrium diagram>
FIG. 11 is an equilibrium diagram of carbon steel showing the carbon content on the horizontal axis. As described above, the carbon content of the rail steel is generally in the range of 0.4 to 1.2 mass%. Rail steel contains Si and Mn in addition to carbon, and in some cases contains reinforcing elements such as Cr. Strictly speaking, the equilibrium diagram changes due to the influence of these elements other than carbon, but the change is negligible in the range of the content (of elements other than carbon) in the rail steel. The hypoeutectoid composition steel has a metal structure mainly composed of pearlite and partially containing ferrite at temperatures below the A1 point, and has a metal structure in which ferrite and austenite are mixed at temperatures from the A1 to A3 points. At a temperature of A3 point or higher, it has an austenite structure.

In the case of a eutectoid composition steel, it has a pearlite structure at a temperature of A1 point or lower, and has an austenite structure at a temperature of Ae point or higher.
A steel with a hypereutectoid composition has a metal structure mainly containing pearlite and partially containing cementite at temperatures below the A1 point, and has a metal structure in which ferrite and cementite are mixed at temperatures between the A1 and Acm points. However, it has an austenite structure at temperatures above the Acm point.

  Any steel having the above composition has a two-phase structure of an austenite phase and a liquid phase at a temperature higher than the higher solidus temperature Ts point, and a liquid phase structure at a temperature higher than the liquidus temperature TL point. In flash butt welding, the temperature at the weld interface reaches the TL point. Moreover, it has a low temperature, so that it goes away from a welding interface.

  Depending on the cooling rate in the natural cooling in the air after rail rolling (natural cooling), or the accelerated cooling that follows the rolling, and the continuous cooling process after reheating after cooling to room temperature. Undercooling occurs from the equilibrium transformation temperature, the content of the proeutectoid phase assumed from the phase diagram is reduced, and the pearlite structure occupies most of the structure fraction. In particular, in the range of 0.6 to 1.0% by mass of carbon in the vicinity of the eutectoid composition, the pearlite structure fraction reaches almost 100%. The “accelerated cooling” refers to forcibly cooling the object to be cooled at a cooling rate faster than the natural cooling rate by ejecting a cooling fluid onto the object to be cooled.

<Welding heat cycle and structural changes>
FIG. 12 is a schematic view showing a structural change accompanying heating / cooling of carbon steel. As shown in FIG. 12, the actual tissue change in the heating process starts at a temperature higher than the equilibrium transformation temperature depending on the heating rate. In addition, the actual structural change in the cooling process starts at a temperature lower than the equilibrium transformation temperature depending on the cooling rate. For this reason, an overheating state occurs in the heating process, and an overcooling state occurs in the cooling process. In general, the transformation temperature in the heating process is distinguished by attaching “c” to the equilibrium transformation temperature such as A1 or A3, and the transformation temperature in the cooling process is distinguished by attaching “r” to the equilibrium transformation temperature such as A1 or A3.

  That is, for the hypoeutectoid composition steel, the starting point of the pearlite → austenite transformation in the heating process is called Ac1, the temperature at which it completely transforms to austenite is called Ac3, the starting point of the austenite → ferrite transformation in the cooling process is Ar3, and austenite. The temperature at which disappears is referred to as Ar1.

  Similarly, for a steel having a hypereutectoid composition, the starting point of the pearlite → austenite transformation in the heating process is called Ac1, the temperature at which it completely transforms to austenite is called Accm, and the starting point of the austenite → cementite transformation in the cooling process is Arcm, The temperature at which austenite disappears is called Ar1.

For the eutectoid composition steel, the starting point of the pearlite → austenite transformation in the heating process is called Ac1, the temperature at which it completely transforms to austenite is called Ace, and the starting point of the austenite → pearlite transformation in the cooling process is Are, austenite is The temperature at which it disappears is called Ar1.
In addition, as shown in FIG. 11, regarding the steel with a eutectoid composition, the point where the A3 line and the Acm line gather is referred to as an Ae point.

<Explanation of tissue change by continuous cooling diagram (CCT diagram)>
Generally, regarding the phase change in the cooling process, the transformation temperature and the precipitation phase differ depending on the steel components and the cooling rate. FIG. 13A to FIG. 13C are CCT diagrams showing the structural change of high carbon steel by continuous cooling.

  FIG. 13A is a CCT diagram of hypoeutectoid steel. When performing slow cooling as shown by the curve (0), pro-eutectoid ferrite precipitates at the temperature at the intersection of the Fs line and the curve (0). Thereafter, the pearlite transformation starts at the temperature of the intersection of the Ps line and the curve (0). Further, the pearlite transformation is completed at the temperature at the intersection of the Pf line and the curve (0). In this case, the metal structure becomes a ferrite pearlite structure containing a small amount of grain boundary ferrite. When the cooling rate is increased and cooling as shown by the curve (1) is performed, the Fs line merges with the Ps line and disappears, so no pro-eutectoid ferrite is precipitated, but the pearlite between the Ps line and the Pf line. Metamorphosis occurs. In this case, the metal structure is a pearlite structure. When the cooling rate is further increased and cooling as shown by the curve (3) is performed, the pearlite transformation stops at the temperature B, and a part of bainite structure may be formed, but the untransformed portion is supercooled with austenite. Is done. A martensitic transformation occurs between the temperature C and the temperature D. In this case, the metal structure is a structure in which pearlite, bainite, and martensite are mixed. When the cooling rate is further increased and cooling as shown by the curve (5) is performed, the curve (5) does not cross the Ps line, and is supercooled to the Ms point with the austenite structure, and then martensitic transformation is performed. Wake up. Since the martensitic structure of high carbon steel is extremely hard and brittle, it is preferable to avoid rapid cooling beyond the cooling curve (2) in rail steel welding.

  FIG. 13B is a CCT diagram of eutectoid steel. In the case of eutectoid steel, proeutectoid ferrite does not precipitate during slow cooling unlike hypoeutectoid steel.

  FIG. 13C is a CCT diagram of hypereutectoid steel. In the case of hypereutectoid steel, proeutectoid cementite precipitates during slow cooling, unlike hypoeutectoid steel where proeutectoid ferrite precipitates during slow cooling. In the figure, the θs line indicates the first precipitation line of cementite. When performing slow cooling where the cooling curve intersects the θs line, the metal structure becomes a cementite / pearlite structure containing a small amount of pro-eutectoid cementite.

<Maximum temperature, structure and hardness from the hardness distribution>
FIG. 14 schematically shows the temperature distribution in the rail axis direction at the completion of rail welding, the metal structure (high temperature structure) at the completion of rail welding, the metal structure after cooling, and the hardness after cooling. The left end of FIG. 14 is a rail base material that is not affected by heat, and the right end indicates the welding center.

  Due to the flushing process, the temperature at the welding center (right end in FIG. 14) exceeds the solidus temperature Ts, and a decarburized portion is generated at the rail welding center. The decarburization part remains thin even after the upset process. Since this part is more prone to proeutectoid ferrite than the surroundings during cooling, the hardness after cooling decreases.

  The first region in the vicinity of the weld center that has been heated beyond Ac3, Ace, or Accm and transformed into a complete austenite phase undergoes pearlite transformation during subsequent cooling, and a uniform hardness is obtained after cooling. Outside the first region is a second region whose temperature is Ac1 or higher but does not exceed Ac3, Ace, or Accm. In the second region, an austenite phase and an untransformed ferrite phase or cementite phase are mixed at the time of heating. The portion transformed into austenite is transformed into pearlite by subsequent cooling, but the untransformed ferrite phase and the undissolved spheroidized cementite remain at room temperature. These structures have lower hardness than normal pearlite transformed from the austenite phase. Since the fraction of the untransformed phase increases as the distance from the welding center increases, the hardness of the second region decreases.

  Further, there is a region that does not reach Ac1 further away from the welding center. Even in this region, in the third region heated to 500 ° C. or higher, the cementite in the pearlite becomes spherical and the hardness decreases. As the distance from the welding center increases, the degree of spheroidization decreases and gradually approaches the hardness of the base metal.

  Further, the macrostructure of the vertical longitudinal section of the welded portion is the same as the base material in the spheroidized region from 500 ° C. to Ac1, but the region of Ac1 or higher, Ac3, Ace, or Accm is the mixed phase region of austenite, ferrite, and cementite. Therefore, it becomes fine and can be clearly discriminated by nitrate alcohol. The first region heated to Ac3, Ace, Accm or more has a tendency to become grainy due to high-temperature heating, but presents a structure close to the base material with the naked eye. In the third region from 500 ° C. to Ac1, spheroidized cementite can be confirmed by a scanning electron microscope (SEM).

  In the rail welding, the distance to which the workpiece is heated to Ac1 or more varies slightly depending on the welding method, the welding conditions, and the location of the rail. As a result of observing the macro structure and hardness distribution in the vertical longitudinal section after rail welding, the rail column portion of flash butt welding was in the range of 10 to 50 mm depending on the welding conditions. Moreover, the distance heated similarly to Ac3, Ae3 or Accm or more was 5-40 mm.

<Generation mechanism of residual stress>
Next, the inventors' idea about the generation mechanism of the remarkable vertical stress in the column part in rail welding will be described.

  In flash butt welding, flushing is caused between the end surfaces of the rails so that the end surfaces reach a melting point of 1300 to 1400 ° C. or higher. On the other hand, the electrode 9 for power supply (see FIG. 2A) is water-cooled in order to suppress wear due to melting or the like. For this reason, the rail material is cooled from the water-cooled electrode 9, and the temperature in the vicinity of the electrode 9 is about 300 ° C. even at the end of welding. The mounting position of the electrode 9 on the rail is usually about 100 mm away from the weld end face. Therefore, when the welding is completed, a temperature difference of about 1000 ° C. occurs between the electrode 9 and the end face distance of about 100 mm. 15A to 15D show the temperature distribution in the column portion of the rail welded portion. Curve XX0 in FIG. 15A shows the temperature distribution immediately after welding. FIG. 15A shows that a steep temperature gradient is generated in the rail material.

  In the thermite welding method, since the rail end face is melted and welded by injecting high-temperature molten steel, a strong temperature distribution is temporarily generated in the rail longitudinal direction by injecting the molten steel.

  In gas pressure welding, the vicinity of the end face of the rail to be welded is heated to around 1000 ° C., and a temperature distribution is generated in the rail longitudinal direction as in the above welding method.

  In Enclosed Arc Welding, welding metal is piled up by manual welding sequentially from the bottom of the rail, taking an operation time of 1 hour or more. The temperature distribution in the longitudinal direction of the rail is generated in the same manner as the above welding method, but the temperature distribution in the vertical direction is slightly different from other welding methods, and the control cooling method of the present invention is not necessarily effective for this welding method. I can not say.

  The occurrence of residual stress in the vertical direction (circumferential direction) in the rail column is most noticeable in flash butt welding with the steepest temperature gradient. In thermite welding and gas pressure welding, the temperature distribution is gradually reduced, that is, the residual stress is relaxed. The present invention is effective for any of these welding methods.

  Residual stress remains as internal stress because the components in the structure constrain shrinkage strain to each other when there is non-uniformity of thermal shrinkage stress due to temperature nonuniformity in the structure. As a result, it occurs. When the structure is at a high temperature, the yield point is low and plastic deformation easily occurs. Therefore, no restraining force is generated between the constituent members, and the residual stress is small. The yield point is known to increase with decreasing temperature, and the occurrence of residual stress increases at low temperatures.

  On the other hand, when transformation occurs from the austenite phase during the cooling process, the crystal lattice is easily recombined in the direction of smaller stress. As a result, the stress may be relieved by a large strain in that direction. For this reason, it is considered that the stress is once released at the transformation point. In view of the state after reaching normal temperature, the stress distribution at a temperature higher than the transformation point may be omitted. However, the temperature distribution itself is important since it is inherited before and after transformation and affects the subsequent generation of residual stress.

  FIG. 15B shows the temperature distribution and shrinkage stress at a certain point in the cooling process in the column of the weld. A solid line XX1 indicates the temperature distribution at that time. Due to the difference between the temperature T1 at the center of the weld and the ambient temperature, shrinkage stress is generated in the weld. Since the stress is once released in the transformation temperature range, the stress is small in that temperature range, and it is considered that residual stress is generated in earnest after T1 is cooled to the transformation completion temperature Ar1.

  FIG. 15C shows the temperature distribution at a certain point in the natural cooling process and the accelerated cooling process in the column part of the welded part. A curve YY2 shown by a broken line shows a temperature distribution when the high temperature region near the weld center is accelerated and cooled. A curved line XX2 indicated by a solid line shows a temperature distribution when the high temperature region near the weld center is naturally cooled.

  FIG. 15D is a diagram illustrating a temperature distribution at a time point when the temperature of the welding center is slightly higher than Ar1 in the natural cooling process and the accelerated cooling process. A curve XX3 indicated by a solid line shows a temperature distribution in the case of natural cooling. A curved line YY3 indicated by a broken line shows a temperature distribution when a wide area near the center of the weld is accelerated and cooled, and a curved line ZZ3 shown by a broken line shows a temperature distribution when the narrow area near the center of the weld is accelerated and cooled. . The time to reach this temperature is shorter when the weld center is accelerated and cooled. Here, a difference in temperature distribution in a certain region near the welding center, for example, a range (LAc1) in which the maximum heating temperature is equal to or higher than Ac1 in the temperature distribution XX0 immediately after welding, and a difference in residual stress based thereon will be described below. To do. When the welding center is accelerated and cooled, the difference between the maximum temperature and the minimum temperature in the range of LAc1 is smaller than in the case of natural cooling. As a result, the generation of residual stress based on the temperature difference within this range is reduced. In addition, even when considered over a wider range, the shrinkage constraint due to the low temperature part away from the welded part is dispersed over a wide range of the welded part, so that the occurrence of residual stress is reduced. As described above, the effect of reducing the residual stress can be obtained by reducing the difference between the maximum temperature and the minimum temperature within a certain range of the welded portion when a certain time has elapsed after welding. By changing the cooling width, the temperature distribution changes, and in some cases, the temperature may be a concave temperature distribution with a low temperature at the center as shown by a curve ZZ3 in FIG. 15D. The same effect can be obtained if the difference from the temperature is reduced.

  According to the experiments by the present inventors, in the region where the maximum heating temperature of the weld exceeds Ac1, the temperature difference between the maximum temperature and the minimum temperature in the region when a certain time has elapsed after welding is within 50 ° C. The effect of reducing the residual stress in the column was confirmed.

  The temperature distribution is affected by the cooling time and the cooling rate. Since rail steel has a high carbon composition, it has high hardenability, and when accelerated cooling is performed from the austenite region, consideration must be given to the variation formula. When the cooling rate is too high, it does not pass through the austenite → pearlite transformation region shown in FIGS. 13A to 13C but passes through the supercooled austenite region on the short time side. For this reason, a hard and brittle martensite structure is generated, and the welded portion becomes brittle. Therefore, in the present invention, in order to prevent embrittlement of the rail steel, the cooling rate is regulated to a maximum of 5 ° C./s. According to the experiments by the present inventors, the cooling time and the cooling width are the main factors for the residual stress in the range of the cooling rate in which martensite does not occur. An appropriate range of the cooling time and the cooling width will be described later.

  The effect of reducing the residual stress by flattening the temperature distribution by accelerating cooling near the weld center is considered to be most effective when the flattened temperature distribution is obtained near Ar1. It is effective even at temperatures above or below. However, even if a flat temperature distribution is obtained in a state where the center temperature of the welded portion is below 200 ° C., the residual stress has already been greatly generated and the effect is small.

<Cooling width of weld zone>
FIG. 16A schematically shows the temperature distribution of the rail head portion, the column portion, and the foot portion in the welded portion when the rail column portion is cooled over a wide range. The temperature distribution on the longitudinal direction BB ′ in the central part of the rail column part only decreases the temperature as a whole, and the function of flattening the temperature distribution in the central part and relaxing the stress cannot be expected. On the other hand, in the temperature distribution diagram of the welding center, as the cooling time becomes longer, the column part relatively lower in temperature than the head and foot, so that the contraction stress in the longitudinal direction of the head and foot Restrained by the cooled column, tensile stress is generated in the longitudinal direction, particularly in the sole. Tension of the residual stress in the longitudinal direction of the sole portion is not preferable because there is a concern that the bending fatigue strength is lowered. However, since the column portion is compressed in the longitudinal direction and the residual stress in the vertical direction (circumferential direction) is relieved, the fatigue strength is improved only in the column portion. Thus, the influence of the cooling width varies depending on the cooling time. The appropriate conditions will be described later.

<Foot cooling>
FIG. 16B shows a temperature distribution when the rail sole is excessively cooled. When the temperature of the foot portion is lower than that of the column portion due to accelerated cooling, the contraction stress in the longitudinal direction of the rail column portion is constrained by the foot portion where the temperature is further lowered. As a result of this action, a tensile stress in the longitudinal direction is generated in the column part, a tensile stress corresponding to the Poisson's ratio is also generated in the vertical direction (circumferential direction), and the vertical direction (circumferential direction) stress of the column part is changed to the tensile side. It becomes. For this reason, it is desirable to keep the temperature higher than that of the rail column when the rail foot is accelerated and cooled for the purpose of increasing the strength.

<Cooling device>
The cooling device for the welded portion is not particularly limited as long as it can appropriately cool the rail portion to be cooled. Although the cooling capacity varies depending on the cooling medium, the type of the cooling medium is not particularly limited as long as the cooling rate defined in the present invention can be obtained. However, it is necessary to be able to adjust the cooling rate for each rail part. For example, when air is used as the cooling medium, it is necessary to be able to adjust the cooling rate by adjusting the ejection amount, the distance between the ejection nozzle and the rail surface, and the like. Details of such a cooling device will be described later.

<Cooling method (head cooling method for high-strength heat-treated rail)>
By the way, the rail head is worn by contact with the wheels. In particular, in a curved track, wear is promoted by a relative slip generated between the wheel and the rail. In addition, the tendency increases as the train weight increases. For this reason, in order to reduce the exchange frequency of a rail in a curve section, the heat-treated rail which hardened the rail head is often adopted.

  A heat-treated rail having high hardness is manufactured by lowering the transformation temperature by accelerated cooling from a high-temperature austenite state performed in the rail manufacturing process. When the heat-treated rail is welded, the hardness of the austenitic region near the weld center is determined according to the cooling rate after welding. For this reason, the hardness of the welded portion is different from the hardness of the portion of the heat-treated rail where the welding heat is not affected.

  Since the cooling rate in the pearlite transformation temperature range in natural cooling after welding by flash butt welding is usually 1 ° C./s or less, the hardness of the welded portion is often lower than that of the heat-treated rail. For this reason, in the welding of heat-treated rails, it is desirable that the rail head is accelerated and cooled in the temperature range from the austenite region to the completion of pearlite transformation after welding to obtain the same hardness as the base material. In welding methods other than flash butt welding, the cooling rate is even slower, so the hardness of the welded portion further decreases. In order to obtain a weld hardness similar to that of the base material by welding of the heat-treated rail, it is desirable to accelerate and cool the rail head from the austenite decomposition start temperature to the completion of pearlite transformation after welding.

  However, the spheroidized cementite region or the ferrite single-phase region in the portion heated to a temperature range of 500 ° C. or higher and not more than Ac 3, Ace, and Accm by welding does not harden even when accelerated cooling is performed. Therefore, the portion where the hardness can be adjusted by performing accelerated cooling is a region heated to the austenite single phase region in the vicinity of the welding center.

<Cooling temperature range>
The cooling temperature range will be described below with reference to FIGS.

FIG. 17 shows a first cooling pattern for accelerating cooling of the rail column after the rail column completes the pearlite transformation.
The higher the starting temperature for cooling the column portion, the better. However, when cooling at a high cooling speed from a high temperature in which the pearlite transformation is not completed, there is a risk that a martensite structure is generated, which is not desirable.

  The cooling rate of the column part needs to be equal to or higher than the natural cooling rate. The faster the cooling rate, the easier the temperature distribution at the welding center is flattened, and the greater the effect of reducing the residual stress.

  In addition, when the foot portion has a cooling rate exceeding the column portion, contraction stress is generated in the column portion with a delay. As a result, the contraction of the column part is constrained by the foot part, so that the tensile residual stress in the longitudinal direction increases. As a result, a tensile stress corresponding to the Poisson's ratio is generated also in the vertical direction (circumferential direction) of the column portion, which is not preferable because the residual stress in the vertical direction (circumferential direction) deteriorates toward the tensile side. With the first cooling pattern shown in FIG. 17, it is possible to reduce the residual stress in the vertical direction (circumferential direction) of the rail column portion and to keep the longitudinal residual stress in the foot portion compressed.

  18A, 18B, and 18C show a second cooling pattern in which accelerated cooling is started from a state in which the temperature of the column portion of the rail welded portion is in the austenite temperature range.

  FIG. 18A shows an example in which the temperature of the column portion is cooled from the austenite region until pearlite transformation is completed. Increase the fatigue strength by increasing the strength by flattening the temperature distribution in the vicinity of the weld center in advance and accelerating cooling of the weld column before reaching the pearlite transformation temperature where the occurrence of residual stress becomes significant. Can do. In order to obtain these effects, it is necessary to start cooling from the austenite temperature range. Moreover, since it cools to Ar1 or less which completes pearlite transformation, the hardness of a cooling part rises notably.

FIG. 18B is an example in which the temperature of the column portion of the rail welded portion starts accelerated cooling from the austenite temperature range and is cooled to the middle of the pearlite transformation range.
Even in this method, the fatigue distribution due to the effect of increasing the strength by flattening the temperature distribution in the vicinity of the weld center in advance and accelerating cooling of the welded column before reaching the pearlite transformation temperature where the occurrence of residual stress becomes significant. Strength can be increased. In order to obtain these effects, it is necessary to start cooling at least from the austenite temperature range. On the other hand, since the cooling is stopped before the pearlite transformation is completed, the allowance for increasing the hardness is smaller than that shown in FIG. 18A.

FIG. 18C is an example in which the accelerated cooling starts from the austenite temperature range when the temperature of the column portion of the rail welded portion is reached, and the cooling is stopped before entering the pearlite transformation range.
Even in this cooling method, the fatigue strength can be increased by flattening the temperature distribution in the vicinity of the weld center in advance before reaching the pearlite transformation temperature at which the occurrence of residual stress becomes significant. In order to obtain this effect, it is necessary to start cooling at least from the austenite temperature range. Further, in order to aim at flattening of the temperature distribution, it is desirable to cool at least 50 ° C. or more from the start of cooling until the temperature drops. In this case, when the cooling stop temperature is cooled to Ar3 point, Ae point, Acm point or less where the metallurgical driving force of pearlite transformation acts, the hardness increases somewhat, but the increase in hardness is shown in FIG. It is smaller than FIG. 18B. Hardness does not increase when the cooling stop temperature is higher than the Ar3 point, Ae point, and Acm point where the metallurgical driving force of pearlite transformation acts, but in this case as well, the residual stress is improved by flattening the temperature distribution. The

  The cooling rate cannot be achieved by a natural cooling rate. Conversely, when the cooling rate is too fast, the column structure does not undergo pearlite transformation and causes bainite or martensite transformation at a lower temperature. The martensitic structure of high carbon steel is extremely hard and brittle and must be avoided. In addition, the strength of the bainite structure varies depending on the transformation temperature, and the segregated portion of the alloy component has a risk that the transformation is further delayed and the martensite structure is mixed, which is not preferable. In order to prevent structures other than these pearlites, the cooling rate needs to be 5 ° C./s or less.

  FIG. 19 shows a third cooling pattern in which the temperature of the column portion of the rail welded portion starts accelerated cooling from the austenite temperature range, and the column portion is accelerated and cooled even after the column portion completes the pearlite transformation. This method increases the strength of the column by flattening the temperature distribution in the vicinity of the weld center in advance and accelerating cooling of the welded column before reaching the pearlite transformation temperature at which the occurrence of residual stress becomes significant. The fatigue strength can be further increased by further cooling the column portion after the effect and the column portion has completed the pearlite transformation. In order to obtain these effects, it is necessary to start cooling at least from the austenite temperature range. At the end of cooling from the austenite temperature range, it is desirable to cool at least 50 ° C. from the start of cooling in order to achieve a flat temperature distribution. Further, in order to obtain an increase in hardness, it is desirable to cool to Ar3 point, Ae point, Acm point or less where metallurgical driving force of pearlite transformation acts. Cooling from the austenite region may be performed until after completion of pearlite transformation, and then cooling after completion of pearlite may be continuously performed.

  The cooling rate from the austenite region to the completion of the pearlite transformation needs to be higher than the natural cooling rate, but is preferably 5 ° C./s or less in order to avoid martensite structure and bainite structure.

  The cooling rate of the column part after completing the pearlite transformation is equal to or higher than the natural cooling rate, and the faster the cooling rate, the greater the effect of reducing the residual stress.

As described above, in order to prevent the martensite structure, the cooling rate in the pearlite transformation region needs to be 5 ° C./s or less. As another method for preventing the martensite structure, a cooling rate is sufficiently slow in the pearlite transformation temperature range, for example, a natural cooling rate or an accelerated cooling of 2 ° C./s or less is provided, and the completion of the pearlite transformation is awaited. The method is also effective. Regardless of the cooling rate in the temperature range other than the pearlite transformation temperature range, by providing this sufficiently slow cooling period in the pearlite transformation temperature range, the pearlite transformation is completed, and the formation of martensite can be completely suppressed.

  In other words, in this cooling pattern, the cooling period of the weld zone is divided into a front stage, a middle stage, and a rear stage, the middle stage is set to a part of the pearlite transformation temperature range of 650 ° C. to 600 ° C., and the natural cooling rate or 2 ° C. / This is a method of setting a slow cooling rate of s or less. In order to suppress the martensite structure, it is desirable that the middle cooling period be 20 seconds or longer.

20A and 20B show a fourth cooling pattern as an example.
FIG. 20A shows that the temperature of the column part of the rail welded portion is accelerated from the austenite temperature range to the middle of the pearlite transformation temperature range by the cooling in the previous stage, and then the natural cooling rate or a slow cooling rate of 2 ° C./s or less as the cooling in the middle stage. This is an example in which the pearlite transformation of the column part is completed, and then the column part is cooled at a cooling rate higher than the natural cooling rate by subsequent cooling. This method has an effect of increasing the strength of the column part because the cooling temperature section in the previous stage includes a part of the pearlite transformation temperature range. The faster the cooling rate of the post column after completion of the pearlite transformation, the greater the effect of reducing the residual stress.

  FIG. 20B shows the natural cooling rate until the column temperature of the rail welded portion starts accelerated cooling from the austenite temperature range and switches to intermediate cooling in the austenite temperature range, and completes pearlite transformation from the austenite temperature range as intermediate cooling. Alternatively, it is an example in which the cooling is performed slowly at a cooling rate of 2 ° C./s or less, and then the column portion is accelerated and cooled as subsequent cooling. The faster the cooling rate of the column portion after completing the pearlite transformation, the greater the effect of reducing the residual stress.

  Also, in the cooling after the pillar part has undergone pearlite transformation, when the cooling speed of the foot part exceeds the pillar part, the pillar part contracts with a delay, and the shrinkage of the pillar part is restrained by the foot part, and the tensile residual stress in the longitudinal direction As a result, the tensile stress corresponding to the Poisson's ratio is generated in the vertical direction (circumferential direction), which is not preferable. By this method, the residual stress in the vertical direction (circumferential direction) of the rail column portion can be further reduced, and higher fatigue strength can be obtained by increasing the strength of the column portion.

  In the above cooling method, it has been explained that it is necessary that the cooling rate of the foot part after the pillar part has undergone pearlite transformation does not exceed that of the pillar part. From this point of view, in order to further improve the residual stress and obtain higher fatigue strength when the rail such as heavy-duty railway is used in a harsh environment, the rail foot part is used in the cooling process after welding. Must be naturally cooled.

  On the other hand, for rails with heat-treated rail heads for fast-wearing curved tracks, in the cooling process after welding, the temperature range in which the rail heads undergo pearlite transformation is accelerated and cooled to provide the same hardness as the base metal rails. It is desirable to give

  FIG. 21 shows a fifth cooling pattern in which the temperature of the rail head portion and the column portion starts accelerated cooling from the austenite temperature range, and the column portion further accelerates and cools the column portion after completing the pearlite transformation.

  In order to harden the rail head and column, accelerated cooling of the rail head must be started from the austenite temperature range exceeding A3, Ae, or Acm, and at least a part of the temperature range until the pearlite transformation is completed. Need to be cooled. At the end of cooling from the austenite temperature range, it is desirable to cool at least 50 ° C. from the start of cooling in order to achieve a flat temperature distribution. Further, in order to increase the hardness, it is necessary to cool to Ar3 point, Ae point, Acm point or less where the metallurgical driving force of pearlite transformation acts, and in order to obtain more sufficient hardness, Ar1 completes pearlite transformation. It is necessary to cool to the following. Cooling from the austenite region may be performed until after completion of pearlite transformation, and then cooling after completion of pearlite may be continuously performed, but may be interrupted in the middle. The cooling rate of the head and column from the austenite region must be higher than the natural cooling rate, and cannot be hardened. is required. In the heat-treated rail in which the rail head is hardened by this method, it is possible to reduce the residual stress in the vertical direction (circumferential direction) of the rail column portion and to suppress partial uneven wear of the welded portion.

<Relationship between proper weld cooling width and cooling time>
Further, when the welded portion is cooled, the temperature distribution state of the welded portion of the rail column portion changes depending on the elapsed time after welding. Since the residual stress is determined by the temperature distribution of the welded portion, the effective cooling range for reducing the residual stress varies depending on the temperature at which cooling is stopped or the cooling time.

Hereinafter, the situation in which the temperature distribution in the rail longitudinal direction from the welding center changes with the lapse of time after welding will be schematically illustrated with reference to FIGS. 22A to 22C, and the change in residual stress in that case will be described.
22A to 22C, the vertical axis represents temperature, and the horizontal axis represents a dimensionless number obtained by dividing the distance from the welding center by the distance LAc1 at which the material is heated to Ac1 or more. The Ac1 temperature of this material is 730 ° C., and the width heated to Ac1 or higher at the time of welding is 20 mm on one side from the welding center, but the total width extending from the welding center to both sides is 40 mm.

  22A to 22C show natural cooling for 1 minute after welding and then air cooling only the rail column at a cooling rate of 2 ° C./s, immediately after welding, 1 minute after welding, 3 minutes after welding, and 9 after welding. Shows the state of the minute. The solid line shows the state of natural cooling of the column part, and the dotted line shows the temperature of the state of accelerated cooling of the column part. In these examples, the foot is naturally cooled, and the temperature distribution of the foot corresponds to a solid line.

  FIG. 22A shows a temperature distribution when k is 0.1, which is a case where the cooling range L of the rail column portion is extremely narrow. The cooling width is 2 mm on one side from the welding center, and the total cooling width is 4 mm when viewed from the whole welded portion. The width (LAc1) at which the rail column part is equal to or greater than Ac1 is 40 mm, and k is 0.1 when the ratio to this and the cooling range L is k.

  In the stage where the cooling time is short, the temperature distribution of the column part is such that only the temperature at the center of the weld decreases, and the region where the temperature at the center of the weld, that is, the column part becomes Ac1 or higher (the horizontal axis in the figure, the distance from the weld center The difference between the maximum temperature and the minimum temperature in the range of 0 to 0.5 times LAc1) exceeds 50 ° C., and the residual stress in the vertical direction (circumferential direction) of the column portion is not reduced.

  Even if the cooling width is extremely narrow and the cooling is performed for a long time under the condition of 0.1, only the vicinity of the welding center is lowered in temperature, and the temperature of the column portion becomes Ac1 or more (the horizontal axis in the figure, The difference between the maximum temperature and the minimum temperature at a distance from the welding center in the range of 0 to 0.5 times LAc1) is about 100 ° C., and the tensile residual stress is not reduced.

  FIG. 22B shows the temperature distribution when k is 1 when the rail column cooling range L is medium. The cooling width is 20 mm on one side from the welding center, and the total cooling width is 40 mm. The ratio (k) to the total width of 40 mm at which the rail column portion is Ac1 or more is 1.

  Maximum temperature and minimum temperature in the region where the temperature of the welded part of the column part becomes Ac1 or higher from the stage where the cooling time is short (the horizontal axis in the figure, the distance from the welding center is 0 to 0.5 times the range of LAc1) Is within 50 ° C., and the residual stress in the vertical direction (circumferential direction) is reduced.

  Even in the state of cooling for a long time, the temperature range in the region where the temperature of the weld becomes Ac1 or higher (distance from the stop of welding in the figure is 0 to 0.5 times the range of LAc1) is within 50 ° C. The residual stress in the vertical direction (circumferential direction) is reduced.

  FIG. 22C shows the temperature distribution when k is 2 when the cooling range L of the rail column portion is extremely wide. The cooling width is 40 mm on one side from the welding center, and the total cooling width is 80 mm. The ratio k to the width 40 mm at which the rail column portion is equal to or greater than Ac1 is 2.

  In the stage where the cooling time is short, the temperature of the column part decreases uniformly over a wide range, so the tendency of the weld center to remain high remains, and the region where the temperature of the weld center part, that is, the column part becomes Ac1 or higher (from the horizontal axis in the figure, The difference between the maximum temperature and the minimum temperature in the range of 0 to 0.5 times the distance of LAc1 exceeds 50 ° C., and the effect of reducing the residual stress in the vertical direction (circumferential direction) is small.

  On the other hand, when the cooling is performed for a long time, the high temperature portion is preferentially cooled, and the temperature in the center of the weld gradually decreases. For this reason, the difference between the maximum temperature and the minimum temperature in the region where the temperature of the weld center, that is, the column portion is Ac1 or higher (the horizontal axis in the figure, the distance from the weld center is 0 to 0.5 times LAc1) is 50. The residual stress in the vertical direction (circumferential direction) of the column portion is reduced.

  On the other hand, when the region where the temperature difference between the pillar part and the foot part (non-cooled part) is significant spreads, the residual stress in the longitudinal direction of the sole part shifts to the tension side. As shown in FIGS. 22A to 22C, the region where the temperature difference between the pillar portion and the foot portion (non-cooled portion) is remarkable increases as the cooling width increases and as the cooling time increases.

  22A to 22C, when the cooling is performed for a long time in FIG. 22C with a wide cooling range, a region where the temperature difference between the pillar part and the foot part is noticeably widened, and the absolute value of the residual stress in the foot sole longitudinal direction is It became tension.

  As described above, the temperature distribution varies depending on the cooling time in addition to the cooling width, and the resulting residual stress varies. The contents described in FIGS. 22A to 22C are divided into a case of short-time cooling and a case of long-time cooling, and are arranged as shown in FIGS. 23A and 23B when arranged by the cooling width.

  First, in the case of short time cooling shown in FIG. 23A, when the cooling width of the column portion is too narrow, the stress in the vertical direction (circumferential direction) of the column portion does not change. If the cooling width is too wide, the welded part and its surroundings are cooled as a whole during cooling, so that the weld center remains in a high temperature state and residual stress is not reduced. On the other hand, the stress in the longitudinal direction of the sole increases as the cooling width increases, and even when the cooling width is extremely wide, it does not reach tension. From the above, when the cooling time is short, the intermediate cooling width in which the vertical stress (circumferential direction) residual stress of the column portion decreases is within the appropriate range.

  On the other hand, when the cooling time shown in FIG. 23B is long, the residual stress of the column portion decreases as the cooling width increases. When the cooling width is too wide, the region where the temperature difference from the foot is large is wide, the shrinkage strain in the longitudinal direction of the foot acts on the column, and the vertical compression (strain direction) of the column in the vertical direction (circumferential direction) of the column Tensile is reduced. Along with this, the contraction in the longitudinal direction of the sole is constrained by the column part, and the residual stress deteriorates until the absolute value reaches the tensile stress. The appropriate range of the cooling width is until the absolute value of the residual stress in the longitudinal direction of the sole reaches the tension.

  From the above, the appropriate range of the cooling width shifts to a narrow range as the cooling time becomes longer. This is shown in FIG. The vertical axis in FIG. 24 indicates the ratio k between the cooling width L of the column part and the width LAc1 heated to Ac1 or more in the column part, and the horizontal axis indicates the cooling time in minutes. An appropriate cooling range is a range surrounded by straight lines shown in FIGS. 4A and 4B, and shifts to a narrow range as the cooling time increases. The range surrounded by the straight line (a) and the straight line (b) is expressed by the formula (1).

k = −0.1t + 1.48 ± 0.85 (1)
In other words, the range of k is −0.1t + 0.63 ≦ k ≦ −0.1t + 2.33 (2)
Indicated by

  As described above, it is possible to reduce the residual stress in the vertical direction of the column portion by flattening the temperature distribution in the vicinity of the welded portion of the rail column portion. Therefore, it is effective to limit the cooling range to a high temperature region near the weld center.

  If the cooling width is too narrow, the cooling efficiency is lowered and the effect of reducing the residual stress is lowered. Therefore, it is desirable to cool at least a range of 5 mm or more.

  By the above controlled cooling after welding, the vertical residual stress of the column portion in the rail welded portion is reduced, and a good weld joint is obtained in which the longitudinal residual stress of the sole portion is also in the compression range. By reducing the residual stress in the vertical direction of the rail column part to a tension of 350 MPa or less, according to the experiments by the present inventors, generation of horizontal cracks in the column part was not recognized in a fatigue test simulating a heavy load railway. Further, since the residual stress in the longitudinal direction of the sole portion is in the compression range, a sufficient fatigue life was obtained even in the bending fatigue test. Further, these effects can be obtained by suppressing the generation of a hard and brittle martensite structure by adjusting the cooling rate when cooling the pearlite transformation temperature range to make 95% or more of the metal structure a pearlite structure.

As shown in FIGS. 20A and 20B, when the cooling process includes a section with a very low cooling rate, the entire cooling time from the start to the end of cooling becomes longer. According to the study by the present inventors, in such a cooling method, the cooling time used in the equations (1) and (2) is obtained by subtracting the slow cooling time of 2 ° C./s or less from the entire cooling time. It is necessary to use a value.

Next, an embodiment for sufficiently securing the hardness of the rail head and further reducing the residual stress of the column portion will be described below with reference to the drawings. In the following description of the embodiment, as shown in FIG. 28, the top surface 115 of the rail head is “the top of the head”, the head side surface 117 is “the head side”, and the constriction between the head and the column is narrowed. The portion 119 may be referred to as a “neck portion”, the upper corner portion 116 between the crown and the head side portion may be referred to as a “gauge corner portion”, and the lower corner portion 118 between the head side portion and the neck portion may be referred to as a “jaw portion”. .
When accelerating and cooling the head portion and the column portion of the rail welded portion, since the jaw portion is angular, as described later, the cooling speed of the jaw portion becomes faster than other portions. The present inventors have discovered that when the cooling rate of the jaw becomes faster than other parts, the residual stress of the column part does not decrease so much. Therefore, in the present embodiment, when the head and the column portion of the rail welded portion are accelerated and cooled, the cooling speed of the jaw portion is made slower than the cooling rate of the column portion, while ensuring the hardness of the rail head portion sufficiently. , To reduce the residual stress in the column. When the cooling rate of the jaw part is made slower than the cooling rate of the column part, it is presumed that the shrinkage strain of the column part is absorbed by reducing the strength in the vicinity of the jaw part and the residual stress of the column part is reduced.

  Moreover, in the cooling method of the rail welding part which concerns on this embodiment, when the height of the head side part which forms the side surface of the said head is set to Hs, it is lower than the position below 2Hs / 3 from the upper end of the said head side part. It is preferable to accelerate and cool the entire head except for the head region. Thereby, the cooling rate of a jaw part is relieve | moderated and the cooling rate of a jaw part can be made slower than the cooling rate of a pillar part.

  In the rail welding part cooling method according to the present invention, a shielding plate is provided in a head region below a position 2Hs / 3 below the upper end of the head side part, and a cooling fluid is directed toward the head. You may squirt. According to such a configuration, the cooling fluid ejected from the upper end of the head side portion toward the head region below 2Hs / 3 is blocked by the shielding plate. The cooling rate is relaxed, and the cooling rate of the jaws can be made slower than the cooling rate of the pillars. In addition, as a kind of the fluid for cooling, what is necessary is just to be able to select either air, air-water (mixed fluid of air and water), and water according to a cooling rate.

  Further, the cooling device used in the method for cooling the rail welded portion is 2Hs / 3 lower than the upper end of the head side portion, where Hs is the height of the head side portion forming the side surface of the head of the rail welded portion. A head cooling unit for accelerating and cooling the entire head area excluding the head area below the position is provided.

  In the rail welded part cooling apparatus according to the present invention, the head cooling unit includes a jet part for jetting a cooling fluid toward the head part, and a position 2Hs / 3 below the upper end of the head side part. You may provide the shielding board which covers a lower head region.

  Further, in the welded joint according to an embodiment of the present invention, the residual stress in the circumferential direction of the rail section in the column portion is set to 300 MPa or less and the hardness of the head is equal to or higher than Hv320 by the method for cooling the rail welded portion. It is said. Here, “hardness” is Vickers hardness.

  If the residual stress in the circumferential direction of the rail cross section in the column portion exceeds 300 MPa, the fatigue strength of the rail is significantly reduced. Further, when the hardness of the head is less than Hv320, the wear of the rail head becomes severe and the durability of the rail is remarkably lowered. It should be noted that wear is very easy on a curved track of a heavy-duty railway, and a rail having a base material head surface layer hardness of around Hv400 is often used. For this reason, it is preferable that the head surface layer of the rail welded portion has a hardness of about Hv400, which is the same as that of the base material rail.

[Flash butt welding]
The residual stress in the vertical direction at the column portion of the rail welded portion is remarkable in flash butt welding in which the temperature gradient is the steepest. For this reason, in this specification, flash butt welding will be described as an example of a rail joint welding method. In addition, it cannot be overemphasized that the cooling method of the rail welding part which concerns on this invention, and the cooling device used therewith are applicable also to other welding methods, such as thermite welding.

  Schematic diagrams for explaining flash butt welding are shown in FIGS. 34A to 34C. In a first process called a flushing process, an arc is continuously generated between the end surfaces of the rails 111 connected by a voltage applied via the electrode 136 connected to the power source 137 (see FIG. 34A). The part where the arc is generated melts locally, and a part of the molten metal is released to the outside as spatter, but the rest remains on the end surface of the rail 111. In the part melted by the arc, a dent called a crater is generated. The rail 111 is gradually approached, arcs are successively generated at new contact portions, and the rail 111 is gradually shortened by repeated local melting. By continuing the flushing process for several tens of seconds to several minutes, the entire end face of the rail 111 is melted. Further, the vicinity of the end surface of the rail 111 is softened due to a temperature rise. When this state is reached, pressurization is performed in the rail axial direction as shown in FIG. 34B (upset process). By this pressurization called upset, the crater formed on the end surface of the rail 111 is crushed, and the molten metal existing between the end surfaces is pushed out of the welding surface. In the vicinity of the softened end surface, the cross section increases due to plastic deformation, and a weld bead 138 is formed around the weld surface. As shown in FIG. 34C, the weld bead 138 is sheared and removed hot by the trimmer 139 in the high temperature period immediately after welding. This process is called trimming. After trimming, a thin weld bead 138 remains around the weld. The thin weld bead 138 remaining on the rail head is smoothed by grinding with a grinder, but the thin weld bead 138 remaining on the rail column and foot is polished by a grinder or unmaintained by a railway company such as Treatment is different.

[Rail steel]
Rail steel is a hypoeutectoid containing 0.5 to 0.8% by mass of carbon as defined in JIS-E1101 "Special rails for ordinary rails and turnouts" and JIS-E1120 "Heat treatment rails". Alternatively, eutectoid carbon steel is generally used. Recently, rail steel with a hypereutectoid composition containing more than 0.8 mass% of carbon, which has improved wearability, has been widely used for heavy-duty cargo lines in overseas mining railways.

[Generation mechanism of residual stress]
When there is non-uniform shrinkage strain due to non-uniform temperature in the rail, the residual stress is the one that the shrinkage stress generated by each part in the rail restraining the shrinkage strain mutually remains as internal stress. is there. When the rail joint is welded, a large temperature difference occurs between the rail weld and the surroundings. As a result, shrinkage stress is generated in the rail weld and becomes residual stress. Therefore, if the vicinity of the welding center is accelerated and cooled, the temperature distribution in the vicinity of the welding center becomes flat, and the occurrence of residual stress at the welding center is reduced. However, the effect of reducing the residual stress by flattening the temperature distribution by accelerating cooling near the weld center is most likely when the flattened temperature distribution is obtained near Ar1 (temperature at which austenite disappears). Even if the effect is large and a flat temperature distribution is obtained in a state where the center temperature of the rail weld is less than 200 ° C., a large residual stress has already occurred and the effect of reducing the residual stress is small.

[Cooling equipment for rail welds]
As shown in FIG. 29, a rail welded part cooling device 110 according to an embodiment of the present invention (hereinafter simply referred to as a cooling device) 110 is a head of rail welded part 150 after rail 111 is welded. The head cooling unit 120 for accelerating cooling 112 and the column cooling unit 121 for accelerating cooling of the column 113 of the rail welded portion 150, and the cooling unit for accelerating and cooling the foot 114 of the rail welded portion 150 are as follows. Not prepared. The cooling device 110 may have a control device (not shown) described later.

  The head cooling unit 120 includes an ejection portion 123 that ejects a cooling fluid toward the head 112, and a pair of shielding plates 125 disposed on the sides of the head 112 (FIGS. 30A and 30B). reference). The ejection portion 123 has a semi-cylindrical shape that surrounds the top portion 112a and the head side portion 112b of the rail welded portion 150, and an ejection hole 123a that ejects a cooling fluid toward the top portion 112a and the head side portion 112b has an inner peripheral surface. Is provided.

  The pair of shielding plates 125 are long in the rail axis direction, and both end portions in the longitudinal direction have a substantially U-shaped (reverse U-shaped) cross section, and an intermediate portion has a substantially L-shaped (reverse L-shaped) cross section ( (See FIGS. 31, 32, and 33). The upper edge portions of both ends of the pair of shielding plates 125 are connected by a hinge 126, and are placed on the crown 112a (see FIG. 33). The pair of shielding plates 125 rotate in a plane orthogonal to the rail axis with the hinge 126 as a rotation axis, and can be opened and closed. The intermediate portion having a substantially L-shaped (inverted L-shaped) cross section is a head region below the position 2Hs / 3 below the upper end of the head side portion 112b, where Hs is the height of the head side portion 112b. (The lower side Hs / 3 of the head side portion 112b + the jaw portion 112c + the neck portion 112d) is covered (see FIGS. 29 and 31). The upper edge 125a of the intermediate portion of the shielding plate 125 is inclined toward the head side portion 112b so that the cooling fluid does not flow into the head region.

  The column portion cooling unit 121 includes a pair of ejection portions 124 that are disposed to face each other with the column portion 113 of the rail welded portion 150 interposed therebetween, and are provided with ejection holes 124 a that eject a cooling fluid toward the column portion 113. (See FIG. 29).

  A supply pipe 128 for supplying a cooling fluid is connected to the ejection part 123 of the head cooling unit 120 that accelerates and cools the head 112 of the rail welded part 150, and the column part 113 of the rail welded part 150 is accelerated and cooled. A supply pipe 129 for supplying a cooling fluid is connected to each of the ejection portions 124 of the column cooling unit 121 that performs the cooling. The supply pipes 128 and 129 are held by a pedestal 122 formed of a gate-type frame laid across the rail 111.

[Cooling method of rail welded part]
Next, a method for cooling the rail welded portion 150 by the cooling device 110 will be described.
(1) As shown by a broken line in FIG. 33, the pair of shielding plates 125 is opened by rotating the hinge 126 around the hinge 126, and the hinge 126 is provided on the top 112a of the rail welded portion 150. Place the part that has been placed. The pair of shielding plates 125 placed on the top of the head 112a has their tip portions turned downward by their own weight, and is in the state shown by the solid line in FIG. As a result, the head region (lower Hs / 3 + jaw portion 112c + neck portion 112d of the head side portion 112b) below the position 2Hs / 3 below the upper end of the head side portion 112b of the rail welded portion 150 is the shielding plate 125. It will be in the covered state (refer FIG. 31).
(2) The gantry 122 made of a portal frame is installed so as to straddle the rail 111, and the ejection part 123 of the head cooling unit 120 is set so as to surround the head top part 112a and the head side part 112b of the rail welded part 150. At the same time, the column cooling units 121 are arranged opposite to each other with the column 113 of the rail welded portion 150 interposed therebetween.
(3) Until the transformation from the austenite temperature range to the pearlite is completed in the head portion 112 and the column portion 113 of the rail welded portion 150, the ejection portion 123 of the head cooling unit 120 and the ejection portion 124 of the column cooling unit 121. The cooling fluid is ejected from the head 112 to accelerate the head 112 and the column 113. A control device provided in the cooling device 110 is used for the above-described cooling control.

FIG. 35 shows the temperature history when the rail welded portion is accelerated and cooled by the rail welded portion cooling method according to the embodiment of the present invention, FIG. 36 shows the temperature history when the rail welded portion is naturally cooled, and FIG. FIG. 37 shows the temperature history when only the head is accelerated and FIG. 38 shows the temperature history when the head and the column of the rail welded portion are accelerated and cooled by the conventional method. From these figures, the cooling speed of the jaws is the fastest in any case of natural cooling, accelerated cooling of the head only, and accelerated cooling of the head and the column by the conventional method. It can be seen that the cooling rate of the jaw is slower than that of the jaw.
FIG. 39 shows the distribution of residual stress in the circumferential direction of the rail cross-section at the rail welding center when the above cooling methods are carried out. The figure shows that when only the head is accelerated and cooled, the residual stress in the column is greater than in natural cooling, and when the head and column are accelerated and cooled by the conventional method, the column is more than in natural cooling. It can be seen that the residual stress of is reduced. Furthermore, according to this invention, it turns out that the residual stress of a column part falls further than the conventional method.

  Although one embodiment of the present invention has been described above, the present invention is not limited to the configuration described in the above-described embodiment, and is within the scope of matters described in the claims. Other possible embodiments and modifications are also included. For example, in the above embodiment, the shielding plate that shields the cooling fluid ejected from the upper end of the head side portion toward the head region below 2Hs / 3 is provided, but the shielding plate is not provided. The cooling fluid may be ejected from the lower end of the head side portion toward the head region above the position above Hs / 3. Moreover, in the said embodiment, although the ejection part of the head cooling unit is made into the semi-cylinder shape, you may provide the ejection part for head top parts, and the ejection part for head side parts.

<Test method>
(About the fatigue test method of the column)
The fatigue strength evaluation test for the horizontal crack of the column part was performed by the method schematically shown in FIG. A rail welded portion was placed on the surface plate 27, and a load was repeatedly applied from the rail head portion of the welded portion with a pressing jig. The radius of curvature of the pressing jig 28 was 450 mm close to the wheel. Considering that the actual load under heavy load is about 20 tons, the load to be applied was set to 30 tons to promote the experimental speed. When the minimum load in the load repetition is 0 ton, the test piece may be lifted up. The load repetition rate was 2 Hz, and the test was terminated when a crack occurred in the weld. Moreover, when the load repetition number was not broken up to 2 million times, the test was terminated there.

(Foot fatigue test method)
The evaluation test of bending fatigue strength was performed by a three-point bending method. FIG. 26 schematically shows the test method. A rail welded portion cut to 1.5 m was placed in the center of the pedestals 29 and 29 ′ set at a distance of 1 m in an upright posture, and a load was applied to the center portion from the rail head by the pushing jig 30. The radius of curvature of the bases 29, 29 ′ and the portion of the pushing jig 30 in contact with the rail was 100 mmR. The test stress was set at the center of the sole of the rail. The minimum stress was 30 MPa, the maximum stress was 330 MPa, and the stress fluctuation range was 300 MPa. A normal flash butt weld joint has a fatigue life of up to 2 million times in a stress range of 300 MPa. The load repetition rate was 5 Hz, and the test was terminated when a crack occurred in the weld. Moreover, when the load repetition number was non-ruptured up to 2 million times, the test was terminated, and it was judged that the tire had sufficient fatigue performance.

(About used rails)
Table 1 shows the three types of rails used. Rail steel A is a steel type commonly known as a normal rail, and is a hypoeutectoid steel containing 0.65 to 0.75% by weight of carbon. It is a raw material as it is rolled, and the rail head has a Vickers hardness of 260 to 260. 290. Rail steel B is a rail heat-treated after rolling and is a eutectoid steel containing 0.75 to 0.85% by weight of carbon, and has a Vickers hardness of 360 to 400 at a hardness of 5 mm below the surface of the rail head. It was used. Rail steel C is a hypereutectoid steel containing carbon content of 0.85 to 0.95%, and is a rail that is heat-treated after rolling and has a Vickers hardness of 400 to 450 at 5mm below the surface of the rail head. It was used. The rail size used was a general railway size with a metric unit weight of 60 kg / m.

  Examples and Comparative Examples of the present invention are shown in Tables 2 to 8. Three specimens were prepared under the same conditions, one of which investigated residual stress, weld hardness, and metal structure, the second conducted a fatigue life evaluation test of the column, and the third conducted a bending fatigue test. It was. In the table, the type of the rail to be welded, the width in the longitudinal direction LAc1, Ac3, Ace, and the width in the region where the maximum heating temperature of the welded portion is Ac1 or higher, the width in the longitudinal direction of the region where it is greater than or equal to Accm, Longitudinal width, ratio k value of cooling width L and LAc1 of column part, cooling time t, upper and lower limits of appropriate range of k value obtained by equation 1, suitability of whether k value is included in upper and lower limits, cooling Temperature range, residual stress measurement value, weld hardness, and number of cracks in fatigue test. Since the measured value of the decarburized region at the welding center varies, the hardness was measured with a Shore hardness meter on the surface 2 mm from the welding center and converted to Vickers hardness. Residual stress was calculated from the change in strain by cutting out the strain gauge bonded portion. The metal structure was mirror-polished on a cross section perpendicular to the rail longitudinal direction at a position 2 mm from the welding center and 2 mm below the surface, etched with 3% nitric acid alcohol, and observed with a microscope. The tissue fraction of the metal structure was observed at a magnification of 100 times and calculated by the point count method. In Tables 2 to 6, those in which structures other than perlite such as martensite were recognized were entered in the remarks column. In addition, the temperature described in the table is the surface temperature near the center of the weld.

  Further, in flash butt welding, the temperature distribution in the longitudinal direction of the weld varies depending on the time adjustment of the flash process. In the following examples, the range of the maximum heating temperature of the welded portion was changed by adjusting the flushing time.

<Example A> Table 2 shows the natural cooling rate in the region defined below in the longitudinal direction of the rail column after the entire rail column has completed the transformation from austenite to pearlite after flash butt welding of the rail. An embodiment in which cooling is performed at a cooling rate exceeding the cooling rate at the rail foot or higher is shown.
The cooling method at this time is as follows. In the longitudinal region calculated by the product (L) of the rail column width LAc1 and the k value in the table at which the maximum heating temperature of the column in the longitudinal direction of the rail column is equal to or higher than the Ac1 point, compressed air or Cooling is performed by controlling the flow rate and flow rate of compressed air including water droplets with a control device, and the flow rate and flow rate of compressed air are within the longitudinal region (range narrower than the region) of the foot where the maximum heating temperature is Ac1 or higher. The region other than the region was naturally cooled. In short, it is a part of the rail that provides accelerated cooling. Steel grade A in Table 1 was used for the welded rail.

  Examples A1 to A6 are examples in which the cooling rate at the time of cooling the column portion after the column portion has completed the pearlite transformation is variously changed. The pearlite transformation completion temperature was about 600 ° C., the column portion cooling start temperature was 500 ° C., and the cooling end temperature was 200 ° C. Example A4 is an example in which the cooling range in the longitudinal direction is changed.

  In any of the examples, the residual stress in the vertical direction (circumferential direction) of the column portion was lower than that of the as-welded material shown in Comparative Example A1. Accordingly, in the as-welded material of Comparative Example A1, cracks occurred with a short life that did not reach 2,000,000 load repetitions in the column fatigue test, whereas in Examples A1 to A6, 2,000 cracks occurred. Cracks did not occur up to 1,000 times. In addition, the residual stress in the longitudinal direction of the sole portion was in the compression range, and in the bending fatigue test, no crack was generated up to 2,000,000 times and there was no breakage, and comprehensively high fatigue strength was confirmed. All of the metal structures had a pearlite structure of 95% or more.

  On the other hand, in Comparative Example A2, the ratio k value of the cooling width L and LAc1 of the column portion was wider than the appropriate range, the foot portion longitudinal stress became tensile, and the bending fatigue test broke along the way with a short life.

  Further, in Comparative Example A3, the foot cooling rate was faster than that of the column part, the residual stress of the column part did not decrease, and the column part was fractured in the middle with a short life in the fatigue test.

  Comparative Example A4 is an example in which the start timing of cooling is as high as 650 ° C., and cooling was started before completion of the pearlite transformation. In addition, the martensite structure fraction was 10% or more in area ratio because the cooling rate was fast, The hardness of the column became abnormally high. In the fatigue test of the column part, it broke during the short life.

  In Comparative Example A5, the ratio k value of the cooling width L and LAc1 of the column portion was narrower than the appropriate range, and the residual stress in the longitudinal direction of the column portion was tensile, and the column portion fractured in the middle of a short life in a fatigue test.

<Example B>
Table 3 shows that after the rails are flash-butt welded, the transformation from the austenite temperature range where the column temperature is higher than Ae to the pearlite is completed in the following limited region in the longitudinal direction of the rail column portion of the welded portion. The Example which cooled at least one part temperature range with the cooling rate exceeding 5 degree-C / s exceeding a natural cooling rate is shown. The cooling method at this time is as follows.
In the longitudinal region calculated by the product (L) of the rail column width LAc1 and the k value in the table at which the maximum heating temperature of the column in the longitudinal direction of the rail column is equal to or higher than the Ac1 point, compressed air or Cooling is performed by controlling the flow rate and flow rate of compressed air containing water droplets, and the flow rate and flow rate of compressed air are controlled in the longitudinal region of the foot where the maximum heating temperature is at least Ac1 (range narrower than the region). The region other than the region was naturally cooled. In short, it is a part of the rail that provides accelerated cooling.

  The pearlite transformation temperature range is 650 ° C. to 600 ° C. in natural cooling, but when cooling is performed, the transformation temperature changes somewhat depending on the cooling rate. Steel grade A in Table 1 was used for the welded rail.

  Examples B1 to B4 are examples in which the cooling rate and the cooling temperature range when the column portion is cooled from the austenite region are variously changed.

  In any of the examples, the residual stress in the vertical direction (circumferential direction) of the column portion was reduced as compared with the as-welded material shown in Comparative Example A1. Accordingly, cracks did not occur up to 2,000,000 times in the column fatigue test. In addition, the residual stress in the longitudinal direction of the sole portion was in the compression range, and in the bending fatigue test, no crack was generated up to 2,000,000 times and there was no breakage, and comprehensively high fatigue strength was confirmed. All of the metal structures had a pearlite structure of 95% or more. Further, by accelerating and cooling the pearlite transformation region of the rail column part, the hardness of the column part is increased to Hv 350 or more, which is further advantageous in terms of fatigue strength.

  On the other hand, in Comparative Example B1, the cooling rate of the column part exceeded 5 ° C./s, the martensite structure fraction in the column part was 10% or more in area ratio, and the hardness was abnormally high. In the fatigue test of the column part, it broke during the short life.

  In Comparative Example B2, the ratio k value between the cooling width L and LAc1 of the column portion was wider than the appropriate range, the foot portion's longitudinal residual stress was in the tensile region, and fractured halfway with a short life in the bending fatigue test.

  Further, in Comparative Example B3, the end temperature of cooling is as high as 760 ° C., and the amount of decrease in temperature due to cooling is small, so the residual stress is not much different from as-welded, and the hardness is not increased because cooling was terminated before pearlite transformation started, In the fatigue test of the column part, it broke during the short life.

  In Comparative Example B4, the ratio k value between the cooling width L and LAc1 of the column portion was narrower than the appropriate range, the residual stress in the longitudinal direction of the column portion was tensile, and the column portion fractured with a short life in the middle.

<Example C> In Table 4, after welding the rails, the temperature in the column part is changed from the austenite temperature range of A3, Ae, or Acm to pearlite in the following limited region in the longitudinal direction of the rail column part of the welded part. At least a part of the temperature range until the transformation of the steel is cooled at a cooling rate exceeding the natural cooling rate and 5 ° C./s or less, and after the entire rail column has completed the transformation from austenite to pearlite, An embodiment in which cooling is performed at a cooling rate exceeding the cooling rate and at or above the cooling rate of the rail foot will be described.
The cooling method at this time is as follows. In the longitudinal region calculated by the product (L) of the rail column width LAc1 and the k value in the table at which the maximum heating temperature of the column in the longitudinal direction of the rail column is equal to or higher than the Ac1 point, compressed air or Cooling is performed by controlling the flow rate and flow rate of compressed air containing water droplets, and the flow rate and flow rate of compressed air are controlled in the longitudinal region of the foot where the maximum heating temperature is at least Ac1 (range narrower than the region). The region other than the region was naturally cooled. In short, it is a part of the rail that provides accelerated cooling.

  The pearlite transformation temperature range is 650 ° C. to 600 ° C. in natural cooling, but when cooling is performed, the transformation temperature changes somewhat depending on the cooling rate. Normal pearlite transformation is completed at less than 600 ° C. The cooling temperature range after completion of the pearlite transformation was 500 ° C to 200 ° C. Steel grade A in Table 1 was used for the welded rail.

  Examples C1 to C4 are examples in which the column portion changes the cooling temperature range and cooling rate when cooling the pearlite transformation temperature region from the austenite region, and the cooling rate of cooling after completion of the pearlite transformation.

  In any of the examples, the residual stress in the vertical direction (circumferential direction) of the column portion was lower than that of the as-welded material shown in Comparative Example A1. Accordingly, cracks did not occur up to 2,000,000 times in the column fatigue test. Further, the residual stress in the longitudinal direction of the sole portion was compression, and in the bending fatigue test, cracks were not generated up to 2,000,000 times, and comprehensively high fatigue strength was confirmed. All of the metal structures had a pearlite structure of 95% or more. In addition, by accelerating and cooling the pearlite transformation region of the rail column part, the hardness of the column part is increased to Hv 350 or more, which is considered to be further advantageous in terms of fatigue strength.

  On the other hand, in Comparative Example C1, the cooling rate of the column part exceeded 5 ° C./s, the martensite structure fraction of the column part was 10% or more in area ratio, and the hardness of the column part was abnormally high. In the fatigue test of the column part, it broke during the short life.

  Further, in Comparative Example C2, the cooling rate of the foot portion was faster than that of the column portion, the residual stress of the column portion did not decrease, and the column portion was fractured in the middle with a short life in the fatigue test.

  In Comparative Example C3, the ratio k value between the cooling width L and LAc1 of the column portion was wider than the appropriate range, the foot portion's longitudinal residual stress was in the tensile region, and the fracture occurred in the bending fatigue test with a short life.

  In Comparative Example C4, the ratio k value between the cooling width L and LAc1 of the column portion was narrower than the appropriate range, the residual stress in the longitudinal direction of the column portion was tensile, and the column portion fractured with a short life in the middle.

<Example D>
Table 5 shows at least a part of the temperature range until the transformation from the austenite temperature range in which the temperature of the column part exceeds A3, Ae, or Acm to pearlite after the rail is welded, after welding the rail. Cooling at a cooling rate exceeding the natural cooling rate, cooling at least a part of the pearlite transformation temperature range at a natural cooling rate or a cooling rate of 2 ° C / s or less, and the entire rail column portion of the welded portion from austenite to pearlite After the transformation is completed, an embodiment is shown in which the region in the longitudinal direction of the rail column part of the welded part in the previous period is cooled at a cooling rate exceeding the natural cooling rate and at or above the cooling rate of the rail foot.
The cooling method at this time is as follows. In the longitudinal region calculated by the product (L) of the rail column width LAc1 and the k value in the table at which the maximum heating temperature of the column in the longitudinal direction of the rail column is equal to or higher than the Ac1 point, compressed air or Cooling is achieved by controlling the flow rate and flow rate of compressed air containing water droplets, cooling by controlling the flow rate and flow rate of compressed air within the longitudinal region of the foot where the maximum heating temperature is Ac1 or higher, The area other than the area was naturally cooled. In short, it is a part of the rail that provides accelerated cooling.
The pearlite transformation temperature range is 650 ° C. to 600 ° C., and the middle stage cooling includes this temperature range, and in the cooling of the post column after completion of the pearlite transformation at 600 ° C. or less, a temperature drop of 200 ° C. is obtained. I did it. Steel grade B shown in Table 1 was used for the rail to be welded.
In Examples D1 to D4, the column part is cooled from the austenite region to a part of the pearlite transformation temperature region, and then the pearlite transformation is completed by cooling at a cooling rate of 2 ° C./s or less or by natural cooling, and further cooling by the latter step. This is an example in which the column portion is accelerated and cooled. In Example D2, the middle stage cooling was natural cooling.
In Examples D5 and D6, the first stage cooling was performed while the column part was in the austenite temperature region, the austenite temperature range to the completion of pearlite transformation were naturally cooled as the middle stage cooling, and the column part was accelerated and cooled as the second stage cooling. It is an example.
In any of the examples, the residual stress in the vertical direction (circumferential direction) of the column portion was lower than that of the as-welded material shown in Comparative Example A1. Accordingly, cracks did not occur up to 2,000,000 times in the column fatigue test. Further, the residual stress in the longitudinal direction of the sole portion was compression, and in the bending fatigue test, cracks were not generated up to 2,000,000 times, and comprehensively high fatigue strength was confirmed. All the metal structures were 100% pearlite structures.
On the other hand, in Comparative Example D1, the foot cooling rate was faster than that of the column part, the residual stress of the column part did not decrease, and the column part fractured shortly in the fatigue test of the column part.
In Comparative Example D2, the ratio k value between the cooling width L of the column portion and LAc1 was wider than the appropriate range, the foot portion's longitudinal residual stress was in the tensile region, and the fracture occurred in the bending fatigue test with a short life.
In Comparative Example D3, the ratio k value of the cooling width L and LAc1 of the column portion was narrower than the appropriate range, the residual stress in the longitudinal direction of the column portion was tensile, and the column portion fractured with a short life in the middle.

<Example E>
Table 6 shows an example in which the rail feet are naturally cooled in addition to the conditions of Examples A, B, and C. The pearlite transformation temperature range is 650 ° C. to 600 ° C. in natural cooling, but when cooling is performed, the transformation temperature changes somewhat depending on the cooling rate. Normal pearlite transformation is completed at less than 600 ° C. The cooling temperature range of the Example which performed the cooling from the temperature range more than A3, Ae, Acm before pearlite transformation is 800-500 degreeC. Moreover, the cooling temperature range of the Example which performed the cooling after completion of pearlite transformation was 500 to 200 degreeC. Steel grade A in Table 1 was used for the welded rail.

  In any of the examples, the residual stress in the vertical direction (circumferential direction) of the column part is lower than that of the as-welded material shown in Comparative Example A1, and the vertical stress (circumferential direction) residual stress of the column part is On average, it is further reduced than the embodiment. In the column fatigue test, no cracks occurred up to 2,000,000 times. Further, the residual stress in the longitudinal direction of the sole portion was in the compression range, and in the bending fatigue test, no crack was generated up to 2,000,000 times, and comprehensively high fatigue strength was confirmed. All of the metal structures had a pearlite structure of 95% or more.

  On the other hand, in Comparative Example E1, the ratio k value between the cooling width L and LAc1 of the column portion was wider than the appropriate range, the foot portion's longitudinal residual stress became a tensile region, and fractured with a short life in the bending fatigue test.

  In Comparative Example E2, the cooling rate of the column part was high exceeding 5 ° C./s, the martensite structure fraction of the column part was 10% or more in area ratio, and the hardness of the column part was abnormally high. In the fatigue test of the column part, it broke during the short life.

  In Comparative Example E3, the ratio k value between the cooling width L and LAc1 of the column portion was narrower than the appropriate range, the residual stress in the longitudinal direction of the column portion was tensile, and the column portion fractured in the middle with a short life in the fatigue test.

<Example F>
In addition to the conditions of Examples A, B, C, and E, at least a part of the temperature range until the transformation of the rail head portion of the welded portion from the A3, Ae, or Acm austenite temperature range to pearlite is completed. Table 7 shows examples in which the cooling rate was exceeded and the cooling rate was 5 ° C./s or less. The pearlite transformation temperature range is 650 ° C. to 600 ° C. in natural cooling, but when cooling is performed, the transformation temperature changes somewhat depending on the cooling rate. Normal pearlite transformation is completed at less than 600 ° C. The cooling temperature range of the Example which performed the cooling from the temperature range more than A3, Ae, Acm before pearlite transformation was 800-500 degreeC. Moreover, the cooling temperature range of the Example which performed the cooling after completion of pearlite transformation was 500 to 200 degreeC. As the rail to be welded, the eutectoid or hypereutectoid heat-treated rail of steel type B or steel type C shown in Table 1 was used.

  In any of the examples, the residual stress in the vertical direction (circumferential direction) of the column portion was lower than that of the as-welded material shown in Comparative Example A1. Accordingly, cracks did not occur up to 2,000,000 times in the column fatigue test. Further, the residual stress in the longitudinal direction of the sole portion was in the compression range, and in the bending fatigue test, no crack was generated up to 2,000,000 times, and comprehensively high fatigue strength was confirmed. All of the metal structures had a pearlite structure of 95% or more.

  On the other hand, in Comparative Example F1, the cooling rate of the foot portion was faster than that of the column portion, the residual stress of the column portion was not lowered, and the column portion was fractured in the middle with a short life in the fatigue test.

  In Comparative Example F2, the ratio k value between the cooling width L of the column portion and LAc1 was wider than the appropriate range, the foot portion's longitudinal residual stress became a tensile region, and the fracture occurred in the bending fatigue test with a short life.

  In Comparative Example F3, the ratio k value between the cooling width L and LAc1 of the column portion was narrower than the appropriate range, and the residual stress in the longitudinal direction of the column portion was tensile.

<Example G>
Table 8 shows an example in which the rail foot portion is naturally cooled in addition to the conditions of Example D in which a slow cooling period of 2 ° C./s or less is provided in a part of the pearlite transformation temperature range. This is an example in which at least a part of the temperature range from the Ae or Acm austenite temperature range to the completion of transformation to pearlite is cooled at a cooling rate of 5 ° C./s or less exceeding the natural cooling rate. Steel grade C in Table 1 was used for the rail to be welded.
Examples G1 and G2 are examples in which the foot is naturally cooled, Examples G3 and G4 are examples in which the head is accelerated and cooled in a part of the temperature range until the transformation from the austenite temperature range to the pearlite is completed, Example G5 , G6 is an example in which the head is accelerated and cooled in a part of the temperature range until the transformation from the austenite temperature range to the pearlite is completed, and the foot is naturally cooled.
In any of the examples, the residual stress in the vertical direction (circumferential direction) of the column portion was lower than that of the as-welded material shown in Comparative Example A1. Accordingly, cracks did not occur up to 2,000,000 times in the column fatigue test. Further, the residual stress in the longitudinal direction of the sole portion was compression, and in the bending fatigue test, cracks were not generated up to 2,000,000 times, and comprehensively high fatigue strength was confirmed. All metal structures were 100% pearlite structures.
On the other hand, in Comparative Example G1, the foot cooling rate was faster than that of the column part, the residual stress of the column part was not lowered, and the column part fractured with a short life in the fatigue test of the column part.
In Comparative Example G2, the ratio k value between the cooling width L and LAc1 of the column portion was wider than the appropriate range, the foot portion's longitudinal residual stress was in the tensile region, and the fracture occurred in the bending fatigue test with a short life.
In Comparative Example G3, the ratio k value between the cooling width L and LAc1 of the column portion was narrower than the appropriate range, the residual stress in the longitudinal direction of the column portion was tensile, and the column portion fractured with a short life in the middle.

  Next, the cooling test of the rail welded part performed using the cooling device 10 will be described. The rail steel used for the cooling test is an American AREA standard 136 pound rail, and its component ratio is 0.8C-0.4Si-1.0Mn-0.2Cr. Rail joints were welded and joined by flash butt welding to form a welded joint. Air was used as a cooling fluid for accelerated cooling of the rail weld. Table 9 shows the air pressure and flow rate during accelerated cooling.

  Prepare two welded joints each cooled under the same conditions, measure temperature, hardness, and residual stress with one of them (see Fig. 40), and test the fatigue life of the column with the other (see below) Simply called “fatigue test”). The temperature measurement of the rail welded part is a total of 5 points at the center of the top of the head, the chin, the 1/2 height of the column part, the foot surface part, and the center of the sole part at a position 20 mm away from the welding center in the rail axial direction. And measured with a K thermocouple. Further, the hardness of the rail welded portion was measured with a Vickers hardness tester at a position 5 mm below the surface of the top of the head and 5 mm below the surface of the head side at a position 5 mm away from the welding center in the rail axis direction.

  The residual stress is measured by attaching a biaxial strain gauge with a gauge length of 2 mm to both sides of the column (1/2 height position of the column) on the weld center line, and this portion is 5 mm thick x 15 mm wide x 15 mm The residual stress was calculated from the relation between stress and strain using the difference between the strain before cutting and the strain after cutting.

  Moreover, the fatigue test of the column part was performed as follows. A rail welded portion was placed on the surface plate, and a load was repeatedly applied to the head of the rail welded portion by a pushing jig whose tip was an arc-shaped convex portion. The radius of curvature of the arc-shaped convex portion was 450 mm close to the wheel. The load to be applied was set to a maximum of 30 ton considering that the actual load under heavy load is about 20 ton. On the other hand, the minimum load during load repetition was 4 tons. The load repetition rate was 2 Hz, and the test was terminated when a crack occurred in the rail weld.

  Table 10 shows a list of test results. The results of the fatigue test are “GOOD” when the fatigue crack does not occur until the load is repeated 2 million times, “FAIR” when the crack is generated more than 1 million times to less than 2 million times, and 100 The case where the crack occurred less than 10,000 times was indicated as “POOR”. Moreover, the residual stress value in a table | surface is an average value of the residual stress computed from the strain gauge stuck on both surfaces of the column part.

  In Example 11, when the entire head region and the column portion were accelerated and cooled after welding, the cooling rate of the head side portion was weakened and the cooling rate of the jaw portion was adjusted to be equal to or lower than the cooling rate of the column portion. The hardness of the top of the head is the same as that of the base metal rail, and the hardness of the head side is reduced by decreasing the cooling rate of the head side, but is harder than when naturally cooled after welding. The residual stress of the column portion was improved as compared with Comparative Examples 11-13. In the fatigue test, fatigue cracks occurred between 1 million and 2 million load cycles, but the fatigue performance is superior to Comparative Examples 11-13.

  In Example 12, when the entire head region and the column portion were accelerated and cooled after welding, the cooling rate of the column portion was increased and the cooling rate of the jaw portion was adjusted to be equal to or lower than the cooling rate of the column portion. The hardness of the top and the side of the head became the same as that of the base material rail, and the residual stress of the column part was improved as compared with Example 11. In the fatigue test, fatigue cracks occurred between 1 million and 2 million load cycles, but the fatigue performance is superior to Comparative Examples 11-13.

  In Example 13, when the head and the column part were accelerated and cooled after welding, the air ejection holes on the head side part were adjusted to be in the range of 2/3 or more of the upper side of the head side part. Although the jaw portion was not subjected to accelerated cooling, the cooling rate increased compared to the case of Comparative Example 11 where no accelerated cooling was applied to any part. This is due to heat conduction accompanying accelerated cooling of the head side portion and the column portion. The hardness at the top of the head is the same as that of the base metal rail, and the head side is also almost the same as the base metal rail. The residual stress of the column portion was improved as compared with Comparative Examples 11-13. In the fatigue test, fatigue cracks occurred between 1 million and 2 million load cycles, but the fatigue performance is superior to Comparative Examples 11-13.

  In Example 14, when the head and the column part were accelerated and cooled after welding, the air ejection holes on the head side part were adjusted so as to be in the range of 1/2 or more on the upper side of the head side part. The hardness of the top of the head is the same as that of the base metal rail, and the hardness of the head side is lowered due to the reduced cooling rate, but is significantly harder than when naturally cooled after welding. The residual stress of the column portion was further improved as compared with Example 13. In the fatigue test, fatigue cracks did not occur up to 2 million load cycles.

  Example 15 is an example in which the head region below the position 2Hs / 3 below the upper end of the head side portion is covered with a shielding plate when the entire head portion and the column portion are accelerated and cooled after welding. The hardness of the top of the head is the same as that of the base metal rail, and the head side is harder than the case where it is naturally cooled after welding, although the hardness is reduced due to the reduced cooling rate. The residual stress of the column portion was improved as compared with Comparative Examples 11-13. In the fatigue test, fatigue cracks occurred between 1 million and 2 million load cycles, but the fatigue performance is superior to Comparative Examples 11-13.

  The sixteenth embodiment is an improvement of the fifteenth embodiment, in which the gap between the shield plate and the rail is narrowed so that the cooling speed of the jaws is smaller than that of the pillars. Although the hardness of the head side part further decreased, it is harder than when naturally cooled after welding. The residual stress in the column portion was significantly improved as compared with Example 15. In the fatigue test, fatigue cracks did not occur up to 2 million load cycles.

  In Example 17, when the head and the column part are accelerated and cooled after the welding, the air ejection hole of the head side part is adjusted so as to be in the range of 1/2 or more of the upper side of the head side part, and the head side part This is an example in which the head region below the position 2Hs / 3 below the upper end of the head is covered with a shielding plate. The hardness of the top and the side of the head is the same as that of the base material rail. The residual stress of the column portion was significantly improved as compared with Comparative Examples 11-13. In the fatigue test, fatigue cracks did not occur up to 2 million load cycles.

  On the other hand, Comparative Example 11 shows an example of natural cooling after welding. The cooling rate at each measurement position was 0.7 to 0.9 ° C./s. The hardness of the head is low, and the residual stress of the column is in a strong tensile state of about 400 MPa. In the fatigue test, fatigue cracks occurred when the load was repeated less than 1 million times. The comparative example 12 shows the example which acceleratedly cooled the whole head part after welding. The hardness of the head is the same as that of the base metal rail, but the residual stress of the column is worse than that when naturally cooled after welding. In the fatigue test, fatigue cracks occurred when the load was repeated less than 1 million times. The comparative example 13 shows the example which acceleratedly cooled the head whole region and the column part after welding. The hardness of the head is the same as that of the base metal rail, and the residual stress of the column is improved compared to the case of natural cooling after welding. In the fatigue test, fatigue cracks occurred when the load was repeated less than 1 million times.

  ADVANTAGE OF THE INVENTION According to this invention, the rail which the fatigue strength of the welding part improved compared with the past can be manufactured efficiently. For this reason, the present invention has sufficient industrial applicability.

DESCRIPTION OF SYMBOLS 1 ... Rail head part 2 ... Rail pillar part 3 ... Rail foot part 4 ... Rail top part 5 ... Rail foot surface 6 ... Rail foot sole 7 ... Weld part 8 ... Weld bead 9 ... Electrode 10 ... Welded Rail 11 ... Welding bead 12 by upset ... Trimmer 13 ... Power supply 14 ... Thermite welding mold 15 ... Thermite welding crucible 16 ... Thermite welding molten steel 17 ... Gas pressure welding burner 18 ... Gas pressure welding trimmer 19 ... Enclosed arc welding Backing metal 20: Enclosed arc welding side surface metal 21: Enclosed arc welding electrode 22: Fatigue crack 23 ... Brittle crack 24 ... Sleeper 25 ... Wheel 26 ... Fatigue cracks XX, YY, ZZ ... Temperature Distribution curve P ... Load 27 ... Surface plate 28 ... Pushing jig 29, 29 '... Pedestal 30 ... Pushing jig 110 ... Cooling device 111 ... Rail 112 ... Head 112a ... Top 1 12b ... head side part 112c ... jaw part 112d ... neck part 113 ... pillar part 114 ... foot part 120 ... head part cooling unit 121 ... pillar part cooling unit 122 ... pedestal 123, 124 ... ejection part 123a, 124a ... ejection hole 125 ... shielding Plate 125a ... Upper edge 126 ... Hinge 128, 129 ... Supply pipe 136 ... Electrode 137 ... Power source 138 ... Weld bead 139 ... Trimmer 150 ... Rail weld

Claims (14)

A method for cooling a rail weld having an Ac1 region heated to a start temperature Ac1 or higher of a transformation from pearlite to austenite, and an Ac3 region heated to a completion temperature Ac3 or higher of the transformation,
A first column cooling step for cooling the column cooling region in the rail welded portion in a partial temperature range until the transformation from austenite to pearlite is completed;
A second column portion cooling step for cooling the column portion cooling region after the entire column portion in the rail welded portion is transformed into pearlite;
A foot cooling step for cooling the foot in the rail weld;
A head cooling step for cooling the head in the rail weld;
With
When the cooling time of the first column cooling step and the second column cooling step is t minutes, the width L in the rail longitudinal direction centering on the welded portion of the column cooling region is set to the Ac1 region and The k value obtained by dividing by the width LAc1 in the rail longitudinal direction centering on the welded portion in the region consisting of the Ac3 region where the maximum heating temperature immediately after welding is Ac1 or higher is −0.1t + 0.63 ≦ k ≦ −. 0.1t + 2.33
The rail welding part cooling method characterized by satisfy | filling the formula shown by these. In the first column part cooling step, the natural cooling rate is exceeded and the cooling is performed at a cooling rate of 5 ° C./s or less,
The method for cooling a rail welded portion according to claim 1, wherein in the second column portion cooling step, cooling is performed at a cooling rate that exceeds a natural cooling rate and is equal to or higher than a cooling rate of the feet. The method for cooling a rail welded portion according to claim 1, wherein in the second column portion cooling step, cooling is performed at a cooling rate that exceeds a natural cooling rate and is equal to or higher than a cooling rate of the feet. In the first column portion cooling step, the natural cooling rate, greater method of cooling the rail weld according to claim 1, characterized in that cooling in the following cooling rate 5 ° C. / s. The first column portion cooling step includes a first column portion cooling first step that is a cooling step in the austenite temperature region, and a first column portion cooling that is subsequently cooled in a temperature range until the transformation to pearlite is completed. With later processes,
In the first column part cooling first stage process, the natural cooling rate is exceeded , and cooling is performed at a cooling rate that is equal to or higher than the cooling rate of the feet,
In the first step of cooling the first column portion, cooling is performed at a natural cooling rate or a cooling rate of 2 ° C./s or less,
The method for cooling a rail welded portion according to claim 1, wherein in the second column portion cooling step, cooling is performed at a cooling rate that exceeds a natural cooling rate and is equal to or higher than a cooling rate of the feet.   The method for cooling a rail weld according to claim 1, wherein the cooling rate of the foot is a natural cooling rate. In the head cooling step, A3, than the natural cooling rate at least a part of the temperature range from Ae or Acm than the austenite temperature region to complete the transformation to pearlite, the cooling in the following cooling rate 5 ° C. / s The method for cooling a rail welded portion according to claim 1, wherein:   The cooling rate of the lower corner portion of the jaw portion is made slower than the cooling rate of the column portion when the head portion and the column portion are cooled. The cooling method of the rail welding part of description.   When the height of the head side part that forms the side surface of the head is Hs, the entire head area excluding the head area below the position below 2Hs / 3 from the upper end of the head side part is accelerated and cooled. The method for cooling a rail welded portion according to claim 8, wherein the rail welded portion is cooled.   The rail according to claim 9, wherein a shielding plate is provided in a head region below a position 2Hs / 3 below the upper end of the head side portion, and a cooling fluid is jetted toward the head. Cooling method for welds. A rail welded joint cooled using the method for cooling a rail weld according to claim 1,
The column part having a vertical residual stress of 350 MPa or less;
A rail foot portion whose longitudinal residual stress is compressive stress;
The rail weld where 95% or more of the metal structure is a pearlite structure;
A rail welded joint characterized by comprising: A rail welded joint cooled using the method for cooling a rail weld according to claim 8,
The column portion having a vertical residual stress of 300 MPa or less;
The head having a hardness of Hv320 or higher;
A rail welded joint characterized by comprising:   If the height of the head side part forming the side surface of the head part of the rail welded part is Hs, the entire head part excluding the head area below the position 2Hs / 3 below the upper end of the head side part is accelerated and cooled. A cooling device for a rail welded portion, comprising: a head cooling unit that performs the operation. The head cooling unit is
An ejection part for ejecting a cooling fluid toward the head;
A shielding plate covering the head region below the position 2Hs / 3 below the upper end of the head side part;
The rail welding part cooling device according to claim 13, comprising: